Encoder, decoder, encoding method, and decoding method

ABSTRACT

An encoder includes memory and circuitry. The circuitry, using the memory, (i) selects a mode from among a plurality of modes each for deriving a motion vector, and derives a motion vector for a current block via the selected mode, and (ii) performs inter prediction encoding on the current block, using the derived motion vector, via one of a skip mode and a non-skip mode different from the skip mode. The plurality of modes include a plurality of first modes each for predicting the motion vector for the current block based on an encoded block neighboring the current block without encoding information indicating a motion vector into a stream. When a second mode included in the plurality of first modes is selected, the current block is encoded via the non-skip mode regardless of presence or absence of a residual coefficient.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. continuation application of PCT InternationalPatent Application Number PCT/JP2018/023207 filed on Jun. 19, 2018,claiming the benefit of priority of U.S. Provisional Patent ApplicationNo. 62/525,004 filed on Jun. 26, 2017, the entire contents of which arehereby incorporated by reference.

BACKGROUND 1. Technical Field

The present disclosure relates to an encoder, a decoder, an encodingmethod, and a decoding method.

2. Description of the Related Art

H.265 has conventionally been present as a standard for encoding avideo. H. 265 is also referred to as high efficiency video coding(HEVC).

SUMMARY

An encoder according to one aspect of the present disclosure includes:circuitry; and memory. In the encoder, the circuitry, using the memory:selects a mode from among a plurality of modes each for deriving amotion vector, and derives a motion vector for a current block via themode selected; and performs inter prediction encoding on the currentblock, using the motion vector derived, via one of a skip mode and anon-skip mode that is different from the skip mode. The plurality ofmodes include a plurality of first modes each for predicting the motionvector for the current block based on an encoded block neighboring thecurrent block without encoding information indicating a motion vectorinto a stream. When a second mode included in the plurality of firstmodes is selected, the current block is encoded via the non-skip moderegardless of presence or absence of a residual coefficient.

A decoder according to one aspect of the present disclosure includes:circuitry; and memory. In the decoder, the circuitry, using the memory:selects a mode from among a plurality of modes each for deriving amotion vector, and derives a motion vector for a current block via themode selected; and performs inter prediction decoding on the currentblock, using the motion vector derived, via one of a skip mode and anon-skip mode that is different from the skip mode. The plurality ofmodes include a plurality of first modes each for predicting the motionvector for the current block based on a decoded block neighboring thecurrent block without decoding information indicating a motion vectorfrom a stream. When a second mode included in the plurality of firstmodes is selected, the current block is decoded via the non-skip moderegardless of presence or absence of a residual coefficient.

It should be noted that these general and specific aspects may beimplemented using a system, an apparatus, a method, an integratedcircuit, a computer program, or a non-transitory computer-readablerecording medium such as a compact disc read only memory (CD-ROM), orany combination of systems, apparatuses, methods, integrated circuits,computer programs, or recording media.

BRIEF DESCRIPTION OF DRAWINGS

These and other objects, advantages and features of the disclosure willbecome apparent from the following description thereof taken inconjunction with the accompanying drawings that illustrate a specificembodiment of the present disclosure.

FIG. 1 is a block diagram illustrating a functional configuration of anencoder according to Embodiment 1;

FIG. 2 illustrates one example of block splitting according toEmbodiment 1;

FIG. 3 is a chart indicating transform basis functions for eachtransform type;

FIG. 4A illustrates one example of a filter shape used in ALF;

FIG. 4B illustrates another example of a filter shape used in ALF;

FIG. 4C illustrates another example of a filter shape used in ALF;

FIG. 5A illustrates 67 intra prediction modes used in intra prediction;

FIG. 5B is a flow chart for illustrating an outline of a predictionimage correction process performed via OBMC processing;

FIG. 5C is a conceptual diagram for illustrating an outline of aprediction image correction process performed via OBMC processing;

FIG. 5D illustrates one example of FRUC;

FIG. 6 is for illustrating pattern matching (bilateral matching) betweentwo blocks along a motion trajectory;

FIG. 7 is for illustrating pattern matching (template matching) betweena template in the current picture and a block in a reference picture;

FIG. 8 is for illustrating a model assuming uniform linear motion;

FIG. 9A is for illustrating deriving a motion vector of each sub-blockbased on motion vectors of neighboring blocks;

FIG. 9B is for illustrating an outline of a process for deriving amotion vector via merge mode;

FIG. 9C is a conceptual diagram for illustrating an outline of DMVRprocessing;

FIG. 9D is for illustrating an outline of a prediction image generationmethod using a luminance correction process performed via LICprocessing;

FIG. 10 is a block diagram illustrating a functional configuration of adecoder according to Embodiment 1;

FIG. 11 is a flowchart indicating inter prediction processing performedby an encoder according to a first example of Embodiment 1;

FIG. 12 is a flowchart indicating inter prediction processing performedby a decoder according to the first example of Embodiment 1;

FIG. 13 is a diagram indicating a syntax structure according to thefirst example of Embodiment 1;

FIG. 14 is a flowchart indicating inter prediction processing performedby an encoder according to a second example of Embodiment 1;

FIG. 15 is a flowchart indicating inter prediction processing performedby a decoder according to the second example of Embodiment 1;

FIG. 16 is a diagram indicating a syntax structure according to thesecond example of Embodiment 1;

FIG. 17 is a flowchart indicating inter prediction processing performedby an encoder according to a third example of Embodiment 1;

FIG. 18 is a flowchart indicating inter prediction processing performedby a decoder according to the third example of Embodiment 1;

FIG. 19 is a diagram indicating a syntax structure according to thethird example of Embodiment 1;

FIG. 20 is a block diagram illustrating an implementation example of theencoder according to Embodiment 1;

FIG. 21 is a block diagram illustrating an implementation example of thedecoder according to Embodiment 1;

FIG. 22 illustrates an overall configuration of a content providingsystem for implementing a content distribution service;

FIG. 23 illustrates one example of an encoding structure in scalableencoding;

FIG. 24 illustrates one example of an encoding structure in scalableencoding;

FIG. 25 illustrates an example of a display screen of a web page;

FIG. 26 illustrates an example of a display screen of a web page;

FIG. 27 illustrates one example of a smartphone; and

FIG. 28 is a block diagram illustrating a configuration example of asmartphone.

DETAILED DESCRIPTION OF THE EMBODIMENTS

An encoder according to one aspect of the present disclosure isincludes: circuitry; and memory. In the encoder, the circuitry, usingthe memory: selects a mode from among a plurality of modes each forderiving a motion vector, and derives a motion vector for a currentblock via the mode selected; and performs inter prediction encoding onthe current block, using the motion vector derived, via one of a skipmode and a non-skip mode that is different from the skip mode. Theplurality of modes include a plurality of first modes each forpredicting the motion vector for the current block based on an encodedblock neighboring the current block without encoding informationindicating a motion vector into a stream. When a second mode included inthe plurality of first modes is selected, the current block is encodedvia the non-skip mode regardless of presence or absence of a residualcoefficient.

According to this, the encoder is capable of improving codingefficiency. For example, this eliminates the need for transmittinginformation indicating whether the second mode is used in the case wherethe skip mode is used. It is thus possible to improve the codingefficiency.

For example, when a third mode which is included in the plurality offirst modes and different from the second mode is selected, the currentblock may be encoded via the non-skip mode when a residual coefficientis present, and may be encoded via the skip mode when a residualcoefficient is not present.

For example, the second mode may be a mode for predicting a motionvector corresponding to affine transformation, based on the encodedblock neighboring the current block.

According to this, in the case where a mode in which a non-skip mode ishighly likely to be selected as a result of a coefficient having anonzero value being generated is performed, the non-skip mode is usedregardless of the presence or absence of a residual coefficient.Accordingly, it is possible to reduce the influence resulting from theskip mode being not selected.

For example, the third mode may be one of a frame rate up-conversion(FRUC) mode and a merge mode.

For example, using the memory, the circuitry may further encodeinformation indicating the presence or absence of a residual coefficientwhen the current block is encoded via the non-skip mode.

For example, using the memory, the circuitry may further: select whetherto perform, on the current block, a luminance correction process bywhich a luminance average value of a prediction image is corrected usinga correction value that is predicted from a luminance value of theencoded block neighboring the current block; and encode the currentblock via the non-skip mode regardless of the presence or absence of aresidual coefficient when the luminance correction process is performedon the current block.

According to this, in the case where the luminance correction process isperformed, in which the non-skip mode is highly likely to be selected asa result of a coefficient having a nonzero value being generated, thenon-skip mode is used regardless of presence or absence of a residualcoefficient. Accordingly, it is possible to reduce the influenceresulting from the skip mode being not selected.

For example, using the memory, the circuitry may further encodeinformation indicating the presence or absence of a residual coefficientwhen the luminance correction process is performed on the current block.

A decoder according to one aspect of the present disclosure includes:circuitry; and memory. The circuitry, using the memory: selects a modefrom among a plurality of modes each for deriving a motion vector, andderives a motion vector for a current block via the mode selected; andperforms inter prediction decoding on the current block, using themotion vector derived, via one of a skip mode and a non-skip mode thatis different from the skip mode. The plurality of modes include aplurality of first modes each for predicting the motion vector for thecurrent block based on a decoded block neighboring the current blockwithout decoding information indicating a motion vector from a stream.When a second mode included in the plurality of first modes is selected,the current block is decoded via the non-skip mode regardless ofpresence or absence of a residual coefficient.

According to this, the decoder is capable of improving codingefficiency. For example, this eliminates the need for transmittinginformation indicating whether the second mode is used in the case wherethe skip mode is used. It is thus possible to improve the codingefficiency.

For example, when a third mode which is included in the plurality offirst modes and different from the second mode is selected, the currentblock may be decoded via the non-skip mode when a residual coefficientis present, and may be decoded via the skip mode when a residualcoefficient is not present.

For example, the second mode may be a mode for predicting a motionvector corresponding to affine transformation, based on the decodedblock neighboring the current block.

According to this, in the case where a mode in which the non-skip modeis highly likely to be selected as a result of a coefficient having anonzero value being generated is performed, the non-skip mode is usedregardless of the presence or absence of a residual coefficient.Accordingly, it is possible to reduce the influence resulting from theskip mode being not selected.

For example, the third mode may be one of a frame rate up-conversion(FRUC) mode and a merge mode.

For example, using the memory, the circuitry may further decodeinformation indicating the presence or absence of a residual coefficientwhen the current block is decoded via the non-skip mode.

For example, using the memory, the circuitry may further: select whetherto perform, on the current block, a luminance correction process bywhich a luminance average value of a prediction image is corrected usinga correction value that is predicted from a luminance value of thedecoded block neighboring the current block; and decode the currentblock via the non-skip mode regardless of the presence or absence of aresidual coefficient when the luminance correction process is performedon the current block.

According to this, in the case where the luminance correction process isperformed, in which the non-skip mode is highly likely to be selected asa result of a coefficient having a nonzero value being generated, thenon-skip mode is used regardless of the presence or absence of aresidual coefficient. Accordingly, it is possible to reduce theinfluence resulting from the skip mode being not selected.

For example, using the memory, the circuitry may further decodeinformation indicating the presence or absence of a residual coefficientwhen the luminance correction process is performed on the current block.

An encoding method according to one aspect of the present disclosureincludes: selecting a mode from among a plurality of modes each forderiving a motion vector, and deriving a motion vector for a currentblock via the mode selected; and performing inter prediction encoding onthe current block, using the motion vector derived, via one of a skipmode and a non-skip mode that is different from the skip mode. In theencoding method, the plurality of modes include a plurality of firstmodes each for predicting the motion vector for the current block basedon an encoded block neighboring the current block without encodinginformation indicating a motion vector into a stream, and when a secondmode included in the plurality of first modes is selected, the currentblock is encoded via the non-skip mode regardless of presence or absenceof a residual coefficient.

According to this, with the encoding method, it is possible to improvecoding efficiency. For example, this eliminates the need fortransmitting information indicating whether the second mode is used inthe case where the skip mode is used. It is thus possible to improve thecoding efficiency.

A decoding method according to one aspect of the present disclosureincludes selecting a mode from among a plurality of modes each forderiving a motion vector, and deriving a motion vector for a currentblock via the mode selected; and performing inter prediction decoding onthe current block, using the motion vector derived, via one of a skipmode and a non-skip mode that is different from the skip mode. In thedecoding method, the plurality of modes include a plurality of firstmodes each for predicting the motion vector for the current block basedon a decoded block surrounding the current block without decodinginformation indicating a motion vector from a stream, and when a secondmode included in the plurality of first modes is selected, the currentblock is decoded via the non-skip mode regardless of presence or absenceof a residual coefficient.

According to this, with the decoding method, it is possible to improvecoding efficiency. For example, this eliminates the need fortransmitting information indicating whether the second mode is used inthe case where the skip mode is used. It is thus possible to improve thecoding efficiency.

Furthermore, these general and specific aspects may be implemented usinga system, an apparatus, a method, an integrated circuit, a computerprogram, or a non-transitory computer-readable recording medium such asa compact disc read only memory (CD-ROM), or any combination of systems,apparatuses, methods, integrated circuits, computer programs, orrecording media.

Hereinafter, embodiments will be described with reference to thedrawings.

Note that the embodiments described below each show a general orspecific example. The numerical values, shapes, materials, components,the arrangement and connection of the components, steps, order of thesteps, etc., indicated in the following embodiments are mere examples,and therefore are not intended to limit the scope of the claims.Therefore, among the components in the following embodiments, those notrecited in any of the independent claims defining the broadest inventiveconcepts are described as optional components.

Embodiment 1

First, an outline of Embodiment 1 will be presented. Embodiment 1 is oneexample of an encoder and a decoder to which the processes and/orconfigurations presented in subsequent description of aspects of thepresent disclosure are applicable. Note that Embodiment 1 is merely oneexample of an encoder and a decoder to which the processes and/orconfigurations presented in the description of aspects of the presentdisclosure are applicable. The processes and/or configurations presentedin the description of aspects of the present disclosure can also beimplemented in an encoder and a decoder different from those accordingto Embodiment 1.

When the processes and/or configurations presented in the description ofaspects of the present disclosure are applied to Embodiment 1, forexample, any of the following may be performed.

-   -   (1) regarding the encoder or the decoder according to Embodiment        1, among components included in the encoder or the decoder        according to Embodiment 1, substituting a component        corresponding to a component presented in the description of        aspects of the present disclosure with a component presented in        the description of aspects of the present disclosure;    -   (2) regarding the encoder or the decoder according to Embodiment        1, implementing discretionary changes to functions or        implemented processes performed by one or more components        included in the encoder or the decoder according to Embodiment        1, such as addition, substitution, or removal, etc., of such        functions or implemented processes, then substituting a        component corresponding to a component presented in the        description of aspects of the present disclosure with a        component presented in the description of aspects of the present        disclosure;    -   (3) regarding the method implemented by the encoder or the        decoder according to Embodiment 1, implementing discretionary        changes such as addition of processes and/or substitution,        removal of one or more of the processes included in the method,        and then substituting a processes corresponding to a process        presented in the description of aspects of the present        disclosure with a process presented in the description of        aspects of the present disclosure;    -   (4) combining one or more components included in the encoder or        the decoder according to Embodiment 1 with a component presented        in the description of aspects of the present disclosure, a        component including one or more functions included in a        component presented in the description of aspects of the present        disclosure, or a component that implements one or more processes        implemented by a component presented in the description of        aspects of the present disclosure;    -   (5) combining a component including one or more functions        included in one or more components included in the encoder or        the decoder according to Embodiment 1, or a component that        implements one or more processes implemented by one or more        components included in the encoder or the decoder according to        Embodiment 1 with a component presented in the description of        aspects of the present disclosure, a component including one or        more functions included in a component presented in the        description of aspects of the present disclosure, or a component        that implements one or more processes implemented by a component        presented in the description of aspects of the present        disclosure;    -   (6) regarding the method implemented by the encoder or the        decoder according to Embodiment 1, among processes included in        the method, substituting a process corresponding to a process        presented in the description of aspects of the present        disclosure with a process presented in the description of        aspects of the present disclosure; and    -   (7) combining one or more processes included in the method        implemented by the encoder or the decoder according to        Embodiment 1 with a process presented in the description of        aspects of the present disclosure.

Note that the implementation of the processes and/or configurationspresented in the description of aspects of the present disclosure is notlimited to the above examples. For example, the processes and/orconfigurations presented in the description of aspects of the presentdisclosure may be implemented in a device used for a purpose differentfrom the moving picture/picture encoder or the moving picture/picturedecoder disclosed in Embodiment 1. Moreover, the processes and/orconfigurations presented in the description of aspects of the presentdisclosure may be independently implemented. Moreover, processes and/orconfigurations described in different aspects may be combined.

[Encoder Outline]

First, the encoder according to Embodiment 1 will be outlined. FIG. 1 isa block diagram illustrating a functional configuration of encoder 100according to Embodiment 1. Encoder 100 is a moving picture/pictureencoder that encodes a moving picture/picture block by block.

As illustrated in FIG. 1 , encoder 100 is a device that encodes apicture block by block, and includes splitter 102, subtractor 104,transformer 106, quantizer 108, entropy encoder 110, inverse quantizer112, inverse transformer 114, adder 116, block memory 118, loop filter120, frame memory 122, intra predictor 124, inter predictor 126, andprediction controller 128.

Encoder 100 is realized as, for example, a generic processor and memory.In this case, when a software program stored in the memory is executedby the processor, the processor functions as splitter 102, subtractor104, transformer 106, quantizer 108, entropy encoder 110, inversequantizer 112, inverse transformer 114, adder 116, loop filter 120,intra predictor 124, inter predictor 126, and prediction controller 128.Alternatively, encoder 100 may be realized as one or more dedicatedelectronic circuits corresponding to splitter 102, subtractor 104,transformer 106, quantizer 108, entropy encoder 110, inverse quantizer112, inverse transformer 114, adder 116, loop filter 120, intrapredictor 124, inter predictor 126, and prediction controller 128.

Hereinafter, each component included in encoder 100 will be described.

[Splitter]

Splitter 102 splits each picture included in an input moving pictureinto blocks, and outputs each block to subtractor 104. For example,splitter 102 first splits a picture into blocks of a fixed size (forexample, 128×128). The fixed size block is also referred to as codingtree unit (CTU). Splitter 102 then splits each fixed size block intoblocks of variable sizes (for example, 64×64 or smaller), based onrecursive quadtree and/or binary tree block splitting. The variable sizeblock is also referred to as a coding unit (CU), a prediction unit (PU),or a transform unit (TU). Note that in this embodiment, there is no needto differentiate between CU, PU, and TU; all or some of the blocks in apicture may be processed per CU, PU, or TU.

FIG. 2 illustrates one example of block splitting according toEmbodiment 1. In FIG. 2 , the solid lines represent block boundaries ofblocks split by quadtree block splitting, and the dashed lines representblock boundaries of blocks split by binary tree block splitting.

Here, block 10 is a square 128×128 pixel block (128×128 block). This128×128 block 10 is first split into four square 64×64 blocks (quadtreeblock splitting).

The top left 64×64 block is further vertically split into two rectangle32×64 blocks, and the left 32×64 block is further vertically split intotwo rectangle 16×64 blocks (binary tree block splitting). As a result,the top left 64×64 block is split into two 16×64 blocks 11 and 12 andone 32×64 block 13.

The top right 64×64 block is horizontally split into two rectangle 64×32blocks 14 and 15 (binary tree block splitting).

The bottom left 64×64 block is first split into four square 32×32 blocks(quadtree block splitting). The top left block and the bottom rightblock among the four 32×32 blocks are further split. The top left 32×32block is vertically split into two rectangle 16×32 blocks, and the right16×32 block is further horizontally split into two 16×16 blocks (binarytree block splitting). The bottom right 32×32 block is horizontallysplit into two 32×16 blocks (binary tree block splitting). As a result,the bottom left 64×64 block is split into 16×32 block 16, two 16×16blocks 17 and 18, two 32×32 blocks 19 and 20, and two 32×16 blocks 21and 22.

The bottom right 64×64 block 23 is not split.

As described above, in FIG. 2 , block 10 is split into 13 variable sizeblocks 11 through 23 based on recursive quadtree and binary tree blocksplitting. This type of splitting is also referred to as quadtree plusbinary tree (QTBT) splitting.

Note that in FIG. 2 , one block is split into four or two blocks(quadtree or binary tree block splitting), but splitting is not limitedto this example. For example, one block may be split into three blocks(ternary block splitting). Splitting including such ternary blocksplitting is also referred to as multi-type tree (MBT) splitting.

[Subtractor]

Subtractor 104 subtracts a prediction signal (prediction sample) from anoriginal signal (original sample) per block split by splitter 102. Inother words, subtractor 104 calculates prediction errors (also referredto as residuals) of a block to be encoded (hereinafter referred to as acurrent block). Subtractor 104 then outputs the calculated predictionerrors to transformer 106.

The original signal is a signal input into encoder 100, and is a signalrepresenting an image for each picture included in a moving picture (forexample, a luma signal and two chroma signals). Hereinafter, a signalrepresenting an image is also referred to as a sample.

[Transformer]

Transformer 106 transforms spatial domain prediction errors intofrequency domain transform coefficients, and outputs the transformcoefficients to quantizer 108. More specifically, transformer 106applies, for example, a predefined discrete cosine transform (DCT) ordiscrete sine transform (DST) to spatial domain prediction errors.

Note that transformer 106 may adaptively select a transform type fromamong a plurality of transform types, and transform prediction errorsinto transform coefficients by using a transform basis functioncorresponding to the selected transform type. This sort of transform isalso referred to as explicit multiple core transform (EMT) or adaptivemultiple transform (AMT).

The transform types include, for example, DCT-II, DCT-V, DCT-VIII,DST-I, and DST-VII. FIG. 3 is a chart indicating transform basisfunctions for each transform type. In FIG. 3 , N indicates the number ofinput pixels. For example, selection of a transform type from among theplurality of transform types may depend on the prediction type (intraprediction and inter prediction), and may depend on intra predictionmode.

Information indicating whether to apply such EMT or AMT (referred to as,for example, an AMT flag) and information indicating the selectedtransform type is signalled at the CU level. Note that the signaling ofsuch information need not be performed at the CU level, and may beperformed at another level (for example, at the sequence level, picturelevel, slice level, tile level, or CTU level).

Moreover, transformer 106 may apply a secondary transform to thetransform coefficients (transform result). Such a secondary transform isalso referred to as adaptive secondary transform (AST) or non-separablesecondary transform (NSST). For example, transformer 106 applies asecondary transform to each sub-block (for example, each 4×4 sub-block)included in the block of the transform coefficients corresponding to theintra prediction errors. Information indicating whether to apply NSSTand information related to the transform matrix used in NSST aresignalled at the CU level. Note that the signaling of such informationneed not be performed at the CU level, and may be performed at anotherlevel (for example, at the sequence level, picture level, slice level,tile level, or CTU level).

Here, a separable transform is a method in which a transform isperformed a plurality of times by separately performing a transform foreach direction according to the number of dimensions input. Anon-separable transform is a method of performing a collective transformin which two or more dimensions in a multidimensional input arecollectively regarded as a single dimension.

In one example of a non-separable transform, when the input is a 4×4block, the 4×4 block is regarded as a single array including 16components, and the transform applies a 16×16 transform matrix to thearray.

Moreover, similar to above, after an input 4×4 block is regarded as asingle array including 16 components, a transform that performs aplurality of Givens rotations on the array (i.e., a Hypercube-GivensTransform) is also one example of a non-separable transform.

[Quantizer]

Quantizer 108 quantizes the transform coefficients output fromtransformer 106. More specifically, quantizer 108 scans, in apredetermined scanning order, the transform coefficients of the currentblock, and quantizes the scanned transform coefficients based onquantization parameters (QP) corresponding to the transformcoefficients. Quantizer 108 then outputs the quantized transformcoefficients (hereinafter referred to as quantized coefficients) of thecurrent block to entropy encoder 110 and inverse quantizer 112.

A predetermined order is an order for quantizing/inverse quantizingtransform coefficients. For example, a predetermined scanning order isdefined as ascending order of frequency (from low to high frequency) ordescending order of frequency (from high to low frequency).

A quantization parameter is a parameter defining a quantization stepsize (quantization width). For example, if the value of the quantizationparameter increases, the quantization step size also increases. In otherwords, if the value of the quantization parameter increases, thequantization error increases.

[Entropy Encoder]

Entropy encoder 110 generates an encoded signal (encoded bitstream) byvariable length encoding quantized coefficients, which are inputs fromquantizer 108. More specifically, entropy encoder 110, for example,binarizes quantized coefficients and arithmetic encodes the binarysignal.

[Inverse Quantizer]

Inverse quantizer 112 inverse quantizes quantized coefficients, whichare inputs from quantizer 108. More specifically, inverse quantizer 112inverse quantizes, in a predetermined scanning order, quantizedcoefficients of the current block. Inverse quantizer 112 then outputsthe inverse quantized transform coefficients of the current block toinverse transformer 114.

[Inverse Transformer]

Inverse transformer 114 restores prediction errors by inversetransforming transform coefficients, which are inputs from inversequantizer 112. More specifically, inverse transformer 114 restores theprediction errors of the current block by applying an inverse transformcorresponding to the transform applied by transformer 106 on thetransform coefficients. Inverse transformer 114 then outputs therestored prediction errors to adder 116.

Note that since information is lost in quantization, the restoredprediction errors do not match the prediction errors calculated bysubtractor 104. In other words, the restored prediction errors includequantization errors.

[Adder]

Adder 116 reconstructs the current block by summing prediction errors,which are inputs from inverse transformer 114, and prediction samples,which are inputs from prediction controller 128. Adder 116 then outputsthe reconstructed block to block memory 118 and loop filter 120. Areconstructed block is also referred to as a local decoded block.

[Block Memory]

Block memory 118 is storage for storing blocks in a picture to beencoded (hereinafter referred to as a current picture) for reference inintra prediction. More specifically, block memory 118 storesreconstructed blocks output from adder 116.

[Loop Filter]

Loop filter 120 applies a loop filter to blocks reconstructed by adder116, and outputs the filtered reconstructed blocks to frame memory 122.A loop filter is a filter used in an encoding loop (in-loop filter), andincludes, for example, a deblocking filter (DF), a sample adaptiveoffset (SAO), and an adaptive loop filter (ALF).

In ALF, a least square error filter for removing compression artifactsis applied. For example, one filter from among a plurality of filters isselected for each 2×2 sub-block in the current block based on directionand activity of local gradients, and is applied.

More specifically, first, each sub-block (for example, each 2×2sub-block) is categorized into one out of a plurality of classes (forexample, 15 or 25 classes). The classification of the sub-block is basedon gradient directionality and activity. For example, classificationindex C is derived based on gradient directionality D (for example, 0 to2 or 0 to 4) and gradient activity A (for example, 0 to 4) (for example,C=5D+A). Then, based on classification index C, each sub-block iscategorized into one out of a plurality of classes (for example, 15 or25 classes).

For example, gradient directionality D is calculated by comparinggradients of a plurality of directions (for example, the horizontal,vertical, and two diagonal directions). Moreover, for example, gradientactivity A is calculated by summing gradients of a plurality ofdirections and quantizing the sum.

The filter to be used for each sub-block is determined from among theplurality of filters based on the result of such categorization.

The filter shape to be used in ALF is, for example, a circular symmetricfilter shape. FIG. 4A through FIG. 4C illustrate examples of filtershapes used in ALF. FIG. 4A illustrates a 5×5 diamond shape filter, FIG.4B illustrates a 7×7 diamond shape filter, and FIG. 4C illustrates a 9×9diamond shape filter. Information indicating the filter shape issignalled at the picture level. Note that the signaling of informationindicating the filter shape need not be performed at the picture level,and may be performed at another level (for example, at the sequencelevel, slice level, tile level, CTU level, or CU level).

The enabling or disabling of ALF is determined at the picture level orCU level. For example, for luma, the decision to apply ALF or not isdone at the CU level, and for chroma, the decision to apply ALF or notis done at the picture level. Information indicating whether ALF isenabled or disabled is signalled at the picture level or CU level. Notethat the signaling of information indicating whether ALF is enabled ordisabled need not be performed at the picture level or CU level, and maybe performed at another level (for example, at the sequence level, slicelevel, tile level, or CTU level).

The coefficients set for the plurality of selectable filters (forexample, 15 or 25 filters) is signalled at the picture level. Note thatthe signaling of the coefficients set need not be performed at thepicture level, and may be performed at another level (for example, atthe sequence level, slice level, tile level, CTU level, CU level, orsub-block level).

[Frame Memory]

Frame memory 122 is storage for storing reference pictures used in interprediction, and is also referred to as a frame buffer. Morespecifically, frame memory 122 stores reconstructed blocks filtered byloop filter 120.

[Intra Predictor]

Intra predictor 124 generates a prediction signal (intra predictionsignal) by intra predicting the current block with reference to a blockor blocks in the current picture and stored in block memory 118 (alsoreferred to as intra frame prediction). More specifically, intrapredictor 124 generates an intra prediction signal by intra predictionwith reference to samples (for example, luma and/or chroma values) of ablock or blocks neighboring the current block, and then outputs theintra prediction signal to prediction controller 128.

For example, intra predictor 124 performs intra prediction by using onemode from among a plurality of predefined intra prediction modes. Theintra prediction modes include one or more non-directional predictionmodes and a plurality of directional prediction modes.

The one or more non-directional prediction modes include, for example,planar prediction mode and DC prediction mode defined in theH.265/high-efficiency video coding (HEVC) standard (see H.265 (ISO/IEC23008-2 HEVC (High Efficiency Video Coding)).

The plurality of directional prediction modes include, for example, the33 directional prediction modes defined in the H.265/HEVC standard. Notethat the plurality of directional prediction modes may further include32 directional prediction modes in addition to the 33 directionalprediction modes (for a total of 65 directional prediction modes). FIG.5A illustrates 67 intra prediction modes used in intra prediction (twonon-directional prediction modes and 65 directional prediction modes).The solid arrows represent the 33 directions defined in the H.265/HEVCstandard, and the dashed arrows represent the additional 32 directions.

Note that a luma block may be referenced in chroma block intraprediction. In other words, a chroma component of the current block maybe predicted based on a luma component of the current block. Such intraprediction is also referred to as cross-component linear model (CCLM)prediction. Such a chroma block intra prediction mode that references aluma block (referred to as, for example, CCLM mode) may be added as oneof the chroma block intra prediction modes.

Intra predictor 124 may correct post-intra-prediction pixel values basedon horizontal/vertical reference pixel gradients. Intra predictionaccompanied by this sort of correcting is also referred to as positiondependent intra prediction combination (PDPC). Information indicatingwhether to apply PDPC or not (referred to as, for example, a PDPC flag)is, for example, signalled at the CU level. Note that the signaling ofthis information need not be performed at the CU level, and may beperformed at another level (for example, on the sequence level, picturelevel, slice level, tile level, or CTU level).

[Inter Predictor]

Inter predictor 126 generates a prediction signal (inter predictionsignal) by inter predicting the current block with reference to a blockor blocks in a reference picture, which is different from the currentpicture and is stored in frame memory 122 (also referred to as interframe prediction). Inter prediction is performed per current block orper sub-block (for example, per 4×4 block) in the current block. Forexample, inter predictor 126 performs motion estimation in a referencepicture for the current block or sub-block. Inter predictor 126 thengenerates an inter prediction signal of the current block or sub-blockby motion compensation by using motion information (for example, amotion vector) obtained from motion estimation. Inter predictor 126 thenoutputs the generated inter prediction signal to prediction controller128.

The motion information used in motion compensation is signalled. Amotion vector predictor may be used for the signaling of the motionvector. In other words, the difference between the motion vector and themotion vector predictor may be signalled.

Note that the inter prediction signal may be generated using motioninformation for a neighboring block in addition to motion informationfor the current block obtained from motion estimation. Morespecifically, the inter prediction signal may be generated per sub-blockin the current block by calculating a weighted sum of a predictionsignal based on motion information obtained from motion estimation and aprediction signal based on motion information for a neighboring block.Such inter prediction (motion compensation) is also referred to asoverlapped block motion compensation (OBMC).

In such an OBMC mode, information indicating sub-block size for OBMC(referred to as, for example, OBMC block size) is signalled at thesequence level. Moreover, information indicating whether to apply theOBMC mode or not (referred to as, for example, an OBMC flag) issignalled at the CU level. Note that the signaling of such informationneed not be performed at the sequence level and CU level, and may beperformed at another level (for example, at the picture level, slicelevel, tile level, CTU level, or sub-block level).

Hereinafter, the OBMC mode will be described in further detail. FIG. 5Bis a flowchart and FIG. 5C is a conceptual diagram for illustrating anoutline of a prediction image correction process performed via OBMCprocessing.

First, a prediction image (Pred) is obtained through typical motioncompensation using a motion vector (MV) assigned to the current block.

Next, a prediction image (Pred_L) is obtained by applying a motionvector (MV_L) of the encoded neighboring left block to the currentblock, and a first pass of the correction of the prediction image ismade by superimposing the prediction image and Pred_L.

Similarly, a prediction image (Pred_U) is obtained by applying a motionvector (MV_U) of the encoded neighboring upper block to the currentblock, and a second pass of the correction of the prediction image ismade by superimposing the prediction image resulting from the first passand Pred_U. The result of the second pass is the final prediction image.

Note that the above example is of a two-pass correction method using theneighboring left and upper blocks, but the method may be a three-pass orhigher correction method that also uses the neighboring right and/orlower block.

Note that the region subject to superimposition may be the entire pixelregion of the block, and, alternatively, may be a partial block boundaryregion.

Note that here, the prediction image correction process is described asbeing based on a single reference picture, but the same applies when aprediction image is corrected based on a plurality of referencepictures. In such a case, after corrected prediction images resultingfrom performing correction based on each of the reference pictures areobtained, the obtained corrected prediction images are furthersuperimposed to obtain the final prediction image.

Note that the unit of the current block may be a prediction block and,alternatively, may be a sub-block obtained by further dividing theprediction block.

One example of a method for determining whether to implement OBMCprocessing is by using an obmc_flag, which is a signal that indicateswhether to implement OBMC processing. As one specific example, theencoder determines whether the current block belongs to a regionincluding complicated motion. The encoder sets the obmc_flag to a valueof “1” when the block belongs to a region including complicated motionand implements OBMC processing when encoding, and sets the obmc_flag toa value of “0” when the block does not belong to a region includingcomplication motion and encodes without implementing OBMC processing.The decoder switches between implementing OBMC processing or not bydecoding the obmc_flag written in the stream and performing the decodingin accordance with the flag value.

Note that the motion information may be derived on the decoder sidewithout being signalled. For example, a merge mode defined in theH.265/HEVC standard may be used. Moreover, for example, the motioninformation may be derived by performing motion estimation on thedecoder side. In this case, motion estimation is performed without usingthe pixel values of the current block.

Here, a mode for performing motion estimation on the decoder side willbe described. A mode for performing motion estimation on the decoderside is also referred to as pattern matched motion vector derivation(PMMVD) mode or frame rate up-conversion (FRUC) mode.

One example of FRUC processing is illustrated in FIG. 5D. First, acandidate list (a candidate list may be a merge list) of candidates eachincluding a motion vector predictor is generated with reference tomotion vectors of encoded blocks that spatially or temporally neighborthe current block. Next, the best candidate MV is selected from among aplurality of candidate MVs registered in the candidate list. Forexample, evaluation values for the candidates included in the candidatelist are calculated and one candidate is selected based on thecalculated evaluation values.

Next, a motion vector for the current block is derived from the motionvector of the selected candidate. More specifically, for example, themotion vector for the current block is calculated as the motion vectorof the selected candidate (best candidate MV), as-is. Alternatively, themotion vector for the current block may be derived by pattern matchingperformed in the vicinity of a position in a reference picturecorresponding to the motion vector of the selected candidate. In otherwords, when the vicinity of the best candidate MV is searched via thesame method and an MV having a better evaluation value is found, thebest candidate MV may be updated to the MV having the better evaluationvalue, and the MV having the better evaluation value may be used as thefinal MV for the current block. Note that a configuration in which thisprocessing is not implemented is also acceptable.

The same processes may be performed in cases in which the processing isperformed in units of sub-blocks.

Note that an evaluation value is calculated by calculating thedifference in the reconstructed image by pattern matching performedbetween a region in a reference picture corresponding to a motion vectorand a predetermined region. Note that the evaluation value may becalculated by using some other information in addition to thedifference.

The pattern matching used is either first pattern matching or secondpattern matching. First pattern matching and second pattern matching arealso referred to as bilateral matching and template matching,respectively.

In the first pattern matching, pattern matching is performed between twoblocks along the motion trajectory of the current block in two differentreference pictures. Therefore, in the first pattern matching, a regionin another reference picture conforming to the motion trajectory of thecurrent block is used as the predetermined region for theabove-described calculation of the candidate evaluation value.

FIG. 6 is for illustrating one example of pattern matching (bilateralmatching) between two blocks along a motion trajectory. As illustratedin FIG. 6 , in the first pattern matching, two motion vectors (MV0, MV1)are derived by finding the best match between two blocks along themotion trajectory of the current block (Cur block) in two differentreference pictures (Ref0, Ref1). More specifically, a difference between(i) a reconstructed image in a specified position in a first encodedreference picture (Ref0) specified by a candidate MV and (ii) areconstructed picture in a specified position in a second encodedreference picture (Ref1) specified by a symmetrical MV scaled at adisplay time interval of the candidate MV may be derived, and theevaluation value for the current block may be calculated by using thederived difference. The candidate MV having the best evaluation valueamong the plurality of candidate MVs may be selected as the final MV.

Under the assumption of continuous motion trajectory, the motion vectors(MV0, MV1) pointing to the two reference blocks shall be proportional tothe temporal distances (TD0, TD1) between the current picture (Cur Pic)and the two reference pictures (Ref0, Ref1). For example, when thecurrent picture is temporally between the two reference pictures and thetemporal distance from the current picture to the two reference picturesis the same, the first pattern matching derives a mirror basedbi-directional motion vector.

In the second pattern matching, pattern matching is performed between atemplate in the current picture (blocks neighboring the current block inthe current picture (for example, the top and/or left neighboringblocks)) and a block in a reference picture. Therefore, in the secondpattern matching, a block neighboring the current block in the currentpicture is used as the predetermined region for the above-describedcalculation of the candidate evaluation value.

FIG. 7 is for illustrating one example of pattern matching (templatematching) between a template in the current picture and a block in areference picture. As illustrated in FIG. 7 , in the second patternmatching, a motion vector of the current block is derived by searching areference picture (Ref0) to find the block that best matches neighboringblocks of the current block (Cur block) in the current picture (CurPic). More specifically, a difference between (i) a reconstructed imageof an encoded region that is both or one of the neighboring left andneighboring upper region and (ii) a reconstructed picture in the sameposition in an encoded reference picture (Ref0) specified by a candidateMV may be derived, and the evaluation value for the current block may becalculated by using the derived difference. The candidate MV having thebest evaluation value among the plurality of candidate MVs may beselected as the best candidate MV.

Information indicating whether to apply the FRUC mode or not (referredto as, for example, a FRUC flag) is signalled at the CU level. Moreover,when the FRUC mode is applied (for example, when the FRUC flag is set totrue), information indicating the pattern matching method (first patternmatching or second pattern matching) is signalled at the CU level. Notethat the signaling of such information need not be performed at the CUlevel, and may be performed at another level (for example, at thesequence level, picture level, slice level, tile level, CTU level, orsub-block level).

Here, a mode for deriving a motion vector based on a model assuminguniform linear motion will be described. This mode is also referred toas a bi-directional optical flow (BIO) mode.

FIG. 8 is for illustrating a model assuming uniform linear motion. InFIG. 8 , (v_(x), v_(y)) denotes a velocity vector, and τ₀ and τ₁ denotetemporal distances between the current picture (Cur Pic) and tworeference pictures (Ref₀, Ref₁). (MVx₀, MVy₀) denotes a motion vectorcorresponding to reference picture Ref₀, and (MVx₁, MVy₁) denotes amotion vector corresponding to reference picture Ref₁.

Here, under the assumption of uniform linear motion exhibited byvelocity vector (v_(x), v_(y)), (MVx₀, MVy₀) and (MVx_(i), MVy_(i)) arerepresented as (v_(x)τ₀, v_(y)τ₀) and (−v_(x)τ₁, −v_(y)τ₁),respectively, and the following optical flow equation is given.

MATH. 1

∂I ^((k)) /∂t+v _(x) ∂I ^((k)) /∂x+v _(y) ∂I ^((k)) /∂y=0.  (1)

Here, I^((k)) denotes a luma value from reference picture k (k=0, 1)after motion compensation. This optical flow equation shows that the sumof (i) the time derivative of the luma value, (ii) the product of thehorizontal velocity and the horizontal component of the spatial gradientof a reference picture, and (iii) the product of the vertical velocityand the vertical component of the spatial gradient of a referencepicture is equal to zero. A motion vector of each block obtained from,for example, a merge list is corrected pixel by pixel based on acombination of the optical flow equation and Hermite interpolation.

Note that a motion vector may be derived on the decoder side using amethod other than deriving a motion vector based on a model assuminguniform linear motion. For example, a motion vector may be derived foreach sub-block based on motion vectors of neighboring blocks.

Here, a mode in which a motion vector is derived for each sub-blockbased on motion vectors of neighboring blocks will be described. Thismode is also referred to as affine motion compensation prediction mode.

FIG. 9A is for illustrating deriving a motion vector of each sub-blockbased on motion vectors of neighboring blocks. In FIG. 9A, the currentblock includes 16 4×4 sub-blocks. Here, motion vector v₀ of the top leftcorner control point in the current block is derived based on motionvectors of neighboring sub-blocks, and motion vector v₁ of the top rightcorner control point in the current block is derived based on motionvectors of neighboring blocks. Then, using the two motion vectors v₀ andv₁, the motion vector (v_(x), v_(y)) of each sub-block in the currentblock is derived using Equation 2 below.

$\begin{matrix}{{MATH}.2} &  \\\left\{ \begin{matrix}{v_{x} = {{\frac{\left( {v_{1x} - v_{0x}} \right)}{w}x} - {\frac{\left( {v_{1y} - v_{0y}} \right)}{w}y} + v_{0x}}} \\{v_{y} = {{\frac{\left( {v_{1y} - v_{0y}} \right)}{w}x} + {\frac{\left( {v_{1x} - v_{0x}} \right)}{w}y} + v_{0y}}}\end{matrix} \right. & (2)\end{matrix}$

Here, x and y are the horizontal and vertical positions of thesub-block, respectively, and w is a predetermined weighted coefficient.

Such an affine motion compensation prediction mode may include a numberof modes of different methods of deriving the motion vectors of the topleft and top right corner control points. Information indicating such anaffine motion compensation prediction mode (referred to as, for example,an affine flag) is signalled at the CU level. Note that the signaling ofinformation indicating the affine motion compensation prediction modeneed not be performed at the CU level, and may be performed at anotherlevel (for example, at the sequence level, picture level, slice level,tile level, CTU level, or sub-block level).

[Prediction Controller]

Prediction controller 128 selects either the intra prediction signal orthe inter prediction signal, and outputs the selected prediction signalto subtractor 104 and adder 116.

Here, an example of deriving a motion vector via merge mode in a currentpicture will be given. FIG. 9B is for illustrating an outline of aprocess for deriving a motion vector via merge mode.

First, an MV predictor list in which candidate MV predictors areregistered is generated. Examples of candidate MV predictors include:spatially neighboring MV predictors, which are MVs of encoded blockspositioned in the spatial vicinity of the current block; a temporallyneighboring MV predictor, which is an MV of a block in an encodedreference picture that neighbors a block in the same location as thecurrent block; a combined MV predictor, which is an MV generated bycombining the MV values of the spatially neighboring MV predictor andthe temporally neighboring MV predictor; and a zero MV predictor, whichis an MV whose value is zero.

Next, the MV of the current block is determined by selecting one MVpredictor from among the plurality of MV predictors registered in the MVpredictor list.

Furthermore, in the variable-length encoder, a merge_idx, which is asignal indicating which MV predictor is selected, is written and encodedinto the stream.

Note that the MV predictors registered in the MV predictor listillustrated in FIG. 9B constitute one example. The number of MVpredictors registered in the MV predictor list may be different from thenumber illustrated in FIG. 9B, the MV predictors registered in the MVpredictor list may omit one or more of the types of MV predictors givenin the example in FIG. 9B, and the MV predictors registered in the MVpredictor list may include one or more types of MV predictors inaddition to and different from the types given in the example in FIG.9B.

Note that the final MV may be determined by performing DMVR processing(to be described later) by using the MV of the current block derived viamerge mode.

Here, an example of determining an MV by using DMVR processing will begiven.

FIG. 9C is a conceptual diagram for illustrating an outline of DMVRprocessing.

First, the most appropriate MVP set for the current block is consideredto be the candidate MV, reference pixels are obtained from a firstreference picture, which is a picture processed in the L0 direction inaccordance with the candidate MV, and a second reference picture, whichis a picture processed in the L1 direction in accordance with thecandidate MV, and a template is generated by calculating the average ofthe reference pixels.

Next, using the template, the surrounding regions of the candidate MVsof the first and second reference pictures are searched, and the MV withthe lowest cost is determined to be the final MV. Note that the costvalue is calculated using, for example, the difference between eachpixel value in the template and each pixel value in the regionssearched, as well as the MV value.

Note that the outlines of the processes described here are fundamentallythe same in both the encoder and the decoder.

Note that processing other than the processing exactly as describedabove may be used, so long as the processing is capable of deriving thefinal MV by searching the surroundings of the candidate MV.

Here, an example of a mode that generates a prediction image by usingLIC processing will be given.

FIG. 9D is for illustrating an outline of a prediction image generationmethod using a luminance correction process performed via LICprocessing.

First, an MV is extracted for obtaining, from an encoded referencepicture, a reference image corresponding to the current block.

Next, information indicating how the luminance value changed between thereference picture and the current picture is extracted and a luminancecorrection parameter is calculated by using the luminance pixel valuesfor the encoded left neighboring reference region and the encoded upperneighboring reference region, and the luminance pixel value in the samelocation in the reference picture specified by the MV.

The prediction image for the current block is generated by performing aluminance correction process by using the luminance correction parameteron the reference image in the reference picture specified by the MV.

Note that the shape of the surrounding reference region illustrated inFIG. 9D is just one example; the surrounding reference region may have adifferent shape.

Moreover, although a prediction image is generated from a singlereference picture in this example, in cases in which a prediction imageis generated from a plurality of reference pictures as well, theprediction image is generated after performing a luminance correctionprocess, via the same method, on the reference images obtained from thereference pictures.

One example of a method for determining whether to implement LICprocessing is by using an lic_flag, which is a signal that indicateswhether to implement LIC processing. As one specific example, theencoder determines whether the current block belongs to a region ofluminance change. The encoder sets the lic_flag to a value of “1” whenthe block belongs to a region of luminance change and implements LICprocessing when encoding, and sets the lic_flag to a value of “0” whenthe block does not belong to a region of luminance change and encodeswithout implementing LIC processing. The decoder switches betweenimplementing LIC processing or not by decoding the lic_flag written inthe stream and performing the decoding in accordance with the flagvalue.

One example of a different method of determining whether to implementLIC processing is determining so in accordance with whether LICprocessing was determined to be implemented for a surrounding block. Inone specific example, when merge mode is used on the current block,whether LIC processing was applied in the encoding of the surroundingencoded block selected upon deriving the MV in the merge mode processingmay be determined, and whether to implement LIC processing or not can beswitched based on the result of the determination. Note that in thisexample, the same applies to the processing performed on the decoderside.

[Decoder Outline]

Next, a decoder capable of decoding an encoded signal (encodedbitstream) output from encoder 100 will be described. FIG. 10 is a blockdiagram illustrating a functional configuration of decoder 200 accordingto Embodiment 1. Decoder 200 is a moving picture/picture decoder thatdecodes a moving picture/picture block by block.

As illustrated in FIG. 10 , decoder 200 includes entropy decoder 202,inverse quantizer 204, inverse transformer 206, adder 208, block memory210, loop filter 212, frame memory 214, intra predictor 216, interpredictor 218, and prediction controller 220.

Decoder 200 is realized as, for example, a generic processor and memory.In this case, when a software program stored in the memory is executedby the processor, the processor functions as entropy decoder 202,inverse quantizer 204, inverse transformer 206, adder 208, loop filter212, intra predictor 216, inter predictor 218, and prediction controller220. Alternatively, decoder 200 may be realized as one or more dedicatedelectronic circuits corresponding to entropy decoder 202, inversequantizer 204, inverse transformer 206, adder 208, loop filter 212,intra predictor 216, inter predictor 218, and prediction controller 220.

Hereinafter, each component included in decoder 200 will be described.

[Entropy Decoder]

Entropy decoder 202 entropy decodes an encoded bitstream. Morespecifically, for example, entropy decoder 202 arithmetic decodes anencoded bitstream into a binary signal. Entropy decoder 202 thendebinarizes the binary signal. With this, entropy decoder 202 outputsquantized coefficients of each block to inverse quantizer 204.

[Inverse Quantizer]

Inverse quantizer 204 inverse quantizes quantized coefficients of ablock to be decoded (hereinafter referred to as a current block), whichare inputs from entropy decoder 202. More specifically, inversequantizer 204 inverse quantizes quantized coefficients of the currentblock based on quantization parameters corresponding to the quantizedcoefficients. Inverse quantizer 204 then outputs the inverse quantizedcoefficients (i.e., transform coefficients) of the current block toinverse transformer 206.

[Inverse Transformer]

Inverse transformer 206 restores prediction errors by inversetransforming transform coefficients, which are inputs from inversequantizer 204.

For example, when information parsed from an encoded bitstream indicatesapplication of EMT or AMT (for example, when the AMT flag is set totrue), inverse transformer 206 inverse transforms the transformcoefficients of the current block based on information indicating theparsed transform type.

Moreover, for example, when information parsed from an encoded bitstreamindicates application of NSST, inverse transformer 206 applies asecondary inverse transform to the transform coefficients.

[Adder]

Adder 208 reconstructs the current block by summing prediction errors,which are inputs from inverse transformer 206, and prediction samples,which is an input from prediction controller 220. Adder 208 then outputsthe reconstructed block to block memory 210 and loop filter 212.

[Block Memory]

Block memory 210 is storage for storing blocks in a picture to bedecoded (hereinafter referred to as a current picture) for reference inintra prediction. More specifically, block memory 210 storesreconstructed blocks output from adder 208.

[Loop Filter]

Loop filter 212 applies a loop filter to blocks reconstructed by adder208, and outputs the filtered reconstructed blocks to frame memory 214and, for example, a display device.

When information indicating the enabling or disabling of ALF parsed froman encoded bitstream indicates enabled, one filter from among aplurality of filters is selected based on direction and activity oflocal gradients, and the selected filter is applied to the reconstructedblock.

[Frame Memory]

Frame memory 214 is storage for storing reference pictures used in interprediction, and is also referred to as a frame buffer. Morespecifically, frame memory 214 stores reconstructed blocks filtered byloop filter 212.

[Intra Predictor]

Intra predictor 216 generates a prediction signal (intra predictionsignal) by intra prediction with reference to a block or blocks in thecurrent picture and stored in block memory 210. More specifically, intrapredictor 216 generates an intra prediction signal by intra predictionwith reference to samples (for example, luma and/or chroma values) of ablock or blocks neighboring the current block, and then outputs theintra prediction signal to prediction controller 220.

Note that when an intra prediction mode in which a chroma block is intrapredicted from a luma block is selected, intra predictor 216 may predictthe chroma component of the current block based on the luma component ofthe current block.

Moreover, when information indicating the application of PDPC is parsedfrom an encoded bitstream, intra predictor 216 correctspost-intra-prediction pixel values based on horizontal/verticalreference pixel gradients.

[Inter Predictor]

Inter predictor 218 predicts the current block with reference to areference picture stored in frame memory 214. Inter prediction isperformed per current block or per sub-block (for example, per 4×4block) in the current block. For example, inter predictor 218 generatesan inter prediction signal of the current block or sub-block by motioncompensation by using motion information (for example, a motion vector)parsed from an encoded bitstream, and outputs the inter predictionsignal to prediction controller 220.

Note that when the information parsed from the encoded bitstreamindicates application of OBMC mode, inter predictor 218 generates theinter prediction signal using motion information for a neighboring blockin addition to motion information for the current block obtained frommotion estimation.

Moreover, when the information parsed from the encoded bitstreamindicates application of FRUC mode, inter predictor 218 derives motioninformation by performing motion estimation in accordance with thepattern matching method (bilateral matching or template matching) parsedfrom the encoded bitstream. Inter predictor 218 then performs motioncompensation using the derived motion information.

Moreover, when BIO mode is to be applied, inter predictor 218 derives amotion vector based on a model assuming uniform linear motion. Moreover,when the information parsed from the encoded bitstream indicates thataffine motion compensation prediction mode is to be applied, interpredictor 218 derives a motion vector of each sub-block based on motionvectors of neighboring blocks.

[Prediction Controller]

Prediction controller 220 selects either the intra prediction signal orthe inter prediction signal, and outputs the selected prediction signalto adder 208.

[First Example of Processing Performed by Inter Predictor of Encoder]

FIG. 11 is a flowchart indicating a first example of inter predictionprocessing performed by inter predictor 126 included in encoder 100. Theprocesses illustrated in FIG. 11 are repeatedly performed by a unit ofprediction block that is a processing unit of inter-picture predictionprocessing.

The inter prediction mode information indicates an inter prediction modethat is used in the inter prediction for a current block that is aprediction block to be processed.

The inter prediction mode can be selected from among a plurality ofmodes which are broadly divided as (i) a method involving encoding of amotion vector (MV) difference and (ii) a method not involving encodingof a motion vector difference.

The method not involving encoding of a motion vector differenceincludes: a merge mode in which a motion vector is obtained by selectingfrom among surrounding encoded blocks; a FRUC mode in which a motionvector is obtained by performing search among encoded regions; and anaffine mode in which, in consideration of affine transformation, amotion vector is obtained for each of sub blocks resulting from dividinga current block.

More specifically, when the inter prediction mode information indicates0 (0 in S101), inter predictor 126 derives a motion vector via the mergemode (S102). When the inter prediction mode information indicates 1 (1in S101), inter predictor 126 derives a motion vector via the FRUC mode(S103). When the inter prediction mode information indicates 2 (2 inS101), inter predictor 126 derives a motion vector via the affine mode(S104). When the inter prediction mode information indicates 3 (3 inS101), inter predictor 126 derives a motion vector via the methodinvolving encoding of a motion vector difference (S111).

With the method not involving encoding of a motion vector difference,inter predictor 126 determines, subsequent to Steps S102, S103, or S104,whether a residual coefficient having a nonzero value is present (S105).When a residual coefficient having a nonzero value is not present (No inS105), inter predictor 126 encodes a current block via the skip mode(S106). On the other hand, when a residual coefficient having a nonzerovalue is present (Yes in S105), inter predictor 126 encodes a currentblock via the non-skip mode (S108).

In both of the case where the skip mode is used and the case where thenon-skip mode is used, inter predictor 126 encodes a luminancecorrection processing signal which indicates whether a luminancecorrection process (LIC processing) for a prediction image is to beapplied (S107 or S109). In addition, since a residual coefficient isalways present in the case of the non-skip mode, inter predictor 126always encodes residual coefficient information which indicates aresidual coefficient, without encoding a residual coefficient presencesignal which indicates presence or absence of a residual coefficient(S110).

On the other hand, with the method involving encoding of a motion vectordifference, subsequent to Step S111, inter predictor 126 always encodesa current block via the non-skip mode (S112). In addition, interpredictor 126 encodes a luminance correction processing signal whichindicates whether a luminance correction process for a prediction imageis to be applied (S113). In addition, since there are cases where aresidual coefficient having a non-zero value is present and where theresidual coefficient having a non-zero value is not present, interpredictor 126 encodes a residual coefficient presence signal (S114).Furthermore, inter predictor 126 encodes the residual coefficientinformation (S110) when a residual coefficient having a nonzero value ispresent (Yes in S115), and does not encode the residual coefficientinformation when the residual coefficient having a nonzero value is notpresent (No in S115).

It should be noted that the skip mode is a mode in which, for example, asignal related to a motion vector difference (e.g., a signal indicatinga motion vector difference) and a signal related to a residualcoefficient (e.g., a signal indicating a residual coefficient) are notencoded. In addition, the non-skip mode is a mode in which, for example,at least one of a signal related to a motion vector difference or asignal related to a residual coefficient may be encoded. Which one ofthe skip mode and the non-skip mode is to be applied may be specified bysyntax such as a “skip_flag”, etc.

[First Example of Processing Performed by Inter Predictor of Decoder]

FIG. 12 is a flowchart illustrating a first example of inter predictionprocessing performed by inter predictor 218 included in decoder 200which decodes a stream generated by encoder 100 and described in FIG. 11. The processes illustrated in FIG. 12 are repeatedly performed by aunit of prediction block that is a processing unit of inter-pictureprediction processing.

The inter prediction mode information indicates an inter prediction modethat is used in the inter prediction for a current block that is aprediction block to be processed.

The inter prediction mode can be selected from among a plurality ofmodes which are broadly divided as (i) a method involving decoding of amotion vector difference and (ii) a method not involving decoding of amotion vector difference.

The method not involving decoding of a motion vector differenceincludes: a merge mode in which a motion vector is obtained by selectingfrom among surrounding decoded blocks; a FRUC mode in which a motionvector is obtained by performing search among decoded regions; and anaffine mode in which, in consideration of affine transformation, amotion vector is obtained for each of sub blocks resulting from dividinga current block.

More specifically, when the inter prediction mode information indicates0 (0 in S201), inter predictor 218 derives a motion vector via the mergemode (S202). When the inter prediction mode information indicates 1 (1in S201), inter predictor 218 derives a motion vector via the FRUC mode(S203). When the inter prediction mode information indicates 2 (2 inS201), inter predictor 218 derives a motion vector via the affine mode(S204). When the inter prediction mode information indicates 3 (3 inS201), inter predictor 218 derives a motion vector via the methodinvolving decoding of a motion vector difference (S211).

With the method not involving decoding of a motion vector difference,subsequent to Steps S202, S203, or S204, inter predictor 218 determineswhether a signal indicating that the skip mode is to be used is decoded(S205). When the signal indicating that the skip mode is to be used isdecoded (Yes in S205), inter predictor 218 decodes the current block viathe skip mode (S206). In the other case (No in S205), inter predictor218 decodes the current block via the non-skip mode (S208).

In both of the case where the skip mode is used and the case where thenon-skip mode is used, inter predictor 218 decodes a luminancecorrection processing signal which indicates whether a luminancecorrection process (LIC processing) for a prediction image is to beapplied (S207 or S209). In addition, since a residual coefficient isalways present in the case of the non-skip mode, inter predictor 218always decodes residual coefficient information which indicates aresidual coefficient, without decoding a residual coefficient presencesignal which indicates presence or absence of a residual coefficient(S210).

On the other hand, when the method involving encoding of a motion vectordifference is used, subsequent to Step S211, inter predictor 218 alwaysdecodes a current block via the non-skip mode (S212). In addition, interpredictor 218 decodes a luminance correction processing signal whichindicates whether a luminance correction process for a prediction imageis to be applied (S213). In addition, since there are cases where aresidual coefficient having a non-zero value is present and where theresidual coefficient having a non-zero value is not present, interpredictor 218 decodes a residual coefficient presence signal (S214).Furthermore, inter predictor 218 decodes the residual coefficientinformation (S210) when a residual coefficient having a nonzero value ispresent (Yes in S215), and does not decode the residual coefficientinformation when the residual coefficient having a nonzero value is notpresent (No in S215).

[Syntax Structure of First Example]

FIG. 13 is a syntax table which indicates a first example of a syntaxstructure of a stream generated by encoder 100 and described in FIG. 11.

First, “skip_flag” specifies whether to use the skip mode or to use thenon-skip mode.

In the case where the skip mode is used, further, “fruc_mode” specifieswhether to use the FRUC mode. In the case where the FRUC mode is notused, further, “affine_flag” specifies whether to use the affine mode.In the case where the affine mode is not used, “merge_idx” forspecifying a surrounding block to be referred to in the merge mode isdescribed. It should be noted that, in any case, “lic_flag” whichindicates whether to apply a luminance correction process for aprediction image is described.

In the case where the non-skip mode is used, “merge_flag” specifieswhether to use a method not involving encoding of a motion vectordifference or to use a method involving encoding of the motion vectordifference.

In the case where the method not involving encoding of a motion vectordifference is used, further, “fruc_mode” specifies whether to use theFRUC mode. In the case where the FRUC mode is not used, further,“affine_flag” specifies whether to use the affine mode. In the casewhere the affine mode is not used, “merge_idx” for specifying asurrounding block to be referred to in the merge mode is described.

In the case where the method involving encoding of a motion vectordifference is used, “MVD” that is information related to the motionvector difference is described.

It should be noted that, in any case, “lic_flag” which indicates whetherto apply a luminance correction process for a prediction image isdescribed. In addition, in the case where the method not involvingencoding of a motion vector difference is not used, “root_cbf” whichindicates whether a residual coefficient having a nonzero value ispresent is described. In addition, in the case where it is indicatedthat the residual coefficient having a nonzero value is present,“residual” that is residual coefficient information is described.

However, when the processing of the first example described withreference to FIG. 11 to FIG. 13 is used, “fruc_mode”, “affine_flag”,“merge_idx”, and “lic_flag” need to be described in the case where theskip mode is selected. With this, there is a possibility of a decreasein coding efficiency due to an increase in the number of items of syntaxdescribed in a stream, when encoding is performed, for example, using alarge number of skip modes under a low-rate encoding condition, etc. Inparticular, in the affine mode and in the luminance correction processfor a prediction image, it is highly likely that a residual coefficienthaving a non-zero value is generated, and thus the adverse effect that“affine_flag” and “lic_flag” have to be described needlessly in the skipmode tends to be increased. In addition, there is also a possibility ofthe adverse effect that circuitry for controlling syntax becomescomplicated.

[Second Example of Processing Performed by Inter Predictor of Encoder]

FIG. 14 is a flowchart indicating a second example of inter predictionprocessing performed by inter predictor 126 included in encoder 100.

In the case where the merge mode or the FRUC mode, which are the modesof the method not involving encoding of a motion vector difference otherthan the affine mode, is used (0 or 1 in S101), in the same manner asthe processes illustrated in FIG. 11 , inter predictor 126: determineswhether a residual coefficient having a nonzero value is present (S105);encodes a current block via the skip mode (S106) when the residualcoefficient having a nonzero value is not present (No in S105); andencodes a current block via the non-skip mode (S108) when the residualcoefficient having a nonzero value is present (Yes in S105). In both ofthe case where the skip mode is used and the case where the non-skipmode is used, inter predictor 126 encodes a luminance correctionprocessing signal (S107 and S109). In addition, in the case of thenon-skip mode, inter predictor 126 always encodes residual coefficientinformation without encoding a residual coefficient presence signal(S110).

In the processes illustrated in FIG. 14 , unlike the processesillustrated in FIG. 11 , an operation equivalent to the operationperformed when the method involving encoding of a motion vectordifference is used is performed, in the case where the affine mode isused.

In other words, in the case where the method involving encoding of amotion vector difference is used and the case where the affine modeamong the modes of the method not involving encoding of a motion vectordifference is used (2 or 3 in S101), inter predictor 126 always encodesa current block via the non-skip mode (S112), and encodes a luminancecorrection processing signal (S113). In addition, inter predictor 126encodes a residual coefficient presence signal (S114), and encodesresidual coefficient information (S110) when a residual coefficienthaving a nonzero value is present (Yes in S115).

[Second Example of Processing Performed by Inter Predictor of Decoder]

FIG. 15 is a flowchart which indicates a second example of interprediction processing performed by inter predictor 218 included indecoder 200 which decodes a stream generated by encoder 100 anddescribed in FIG. 14 .

In the case where the merge mode or the FRUC mode, which are the modesof the method not involving encoding of a motion vector difference otherthan the affine mode, is used (0 or 1 in S201), in the same manner asthe processes illustrated in FIG. 12 , inter predictor 218: decodes acurrent block via the skip mode (S206) in the case where a signalindicating that the skip mode is to be used is decoded (Yes in S205);and decodes a current block via the non-skip mode (S208) in the othercase (No in S205). In addition, in both of the case where the skip modeis used and the case where the non-skip mode is used, inter predictor218 decodes a luminance correction processing signal (S207 and S209).Furthermore, in the case of the non-skip mode, inter predictor 218always decodes residual coefficient information without decoding aresidual coefficient presence signal (S210).

In the processes illustrated in FIG. 15 , unlike the processesillustrated in FIG. 12 , an operation equivalent to the operationperformed when the method involving decoding of a motion vectordifference is used is performed when the affine mode is used.

In other words, in the case where the method involving decoding of amotion vector difference is used and the case where the affine modeamong the modes of the method not involving decoding of a motion vectordifference is used (2 or 3 in S201), inter predictor 218 always decodesa current block via the non-skip mode (S212), and decodes a luminancecorrection processing signal (S213). In addition, inter predictor 218decodes a residual coefficient presence signal (S214), and decodesresidual coefficient information (S210) when a residual coefficient ispresent (Yes in S215).

[Syntax Structure of Second Example]

FIG. 16 is a syntax table which indicates a second example of a syntaxstructure of the stream generated by encoder 100 and described in FIG.14 .

First, “skip_flag” specifies whether to use the skip mode or to use thenon-skip mode.

In the case where the skip mode is used, further, “fruc_mode” specifieswhether to use the FRUC mode. In the case where the FRUC mode is notused, “merge_idx” for specifying a surrounding block to be referred toin the merge mode is described. It should be noted that, in any case,“lic_flag” which indicates whether to apply a luminance correctionprocess for a prediction image is described.

In the case where the non-skip mode is used, “merge_flag” specifieswhether to use the method not involving encoding of a motion vectordifference or to use the method involving encoding of the motion vectordifference.

In the case where the method not involving encoding of a motion vectordifference is used, further, “fruc_mode” specifies whether to use theFRUC mode. In the case where the FRUC mode is not used, further,“affine_flag” specifies whether to use the affine mode. In the casewhere the affine mode is not used, “merge_idx” for specifying asurrounding block to be referred to in the merge mode is described.

In the case where the method involving encoding of a motion vectordifference is used, “MVD” that is information related to the motionvector difference is described.

It should be noted that, in any case, “lic_flag” which indicates whetherto apply a luminance correction process for a prediction image isdescribed. In addition, in the case where the method not involvingencoding of a motion vector difference is not used or the case where theaffine mode among the modes of the method not involving encoding of amotion vector difference is used, “root_cbf” which indicates whether aresidual coefficient having a nonzero value is present is described. Inaddition, in the case where it is indicated that the residualcoefficient having a nonzero value is present, “residual” that isresidual coefficient information is described.

[Advantageous Effect of Second Example]

According to the second example, when the skip mode is selected,“fruc_mode”, “merge_idx”, and “lic_flag” need to be described. Thus,when, for example, encoding is performed using a large number of skipmodes under a low-rate encoding condition, etc., the number of items ofsyntax described in a stream is less than that of the first exampledescribed with reference to FIG. 11 to FIG. 13 . Accordingly, there is apossibility that coding efficiency can be improved. In particular, inthe affine mode, since it is highly likely that a residual coefficienthaving a nonzero value is generated, the advantageous effect ofreduction in the code amount resulting from not describing “affine_flag”in the skip mode tends to be great. In addition, there is a possibilitythat the circuitry for controlling syntax can be more simplified.

It should be noted that not all of the constituent elements described inthe second example are always necessary, and encoder 100 and decoder 200may be provided with only part of the constituent elements described inthe second example.

For example, although four modes respectively associated with 0 to 3 aredescribed as examples of the inter prediction mode information in FIG.14 and FIG. 15 , those numerical numbers and modes are merely examples,and thus other numerical numbers and modes may be used. In particular,although the example describes that three modes, namely, the merge mode,the FRUC mode, and the affine mode are used in the method not involvingencoding of the motion vector difference, at least two modes among thethree modes are sufficient to be used. In addition, a mode other thanthe above-described modes may be used. Furthermore, processingequivalent to the processing described regarding the affine mode may beapplied to a mode different from the affine mode.

In addition, the syntax structure described with reference to FIG. 16 ismerely one example, and part of the syntax in FIG. 16 may be replacedwith other syntax, or deletion or addition may be carried out.

[Third Example of Processing Performed by Inter Predictor of Encoder]

FIG. 17 is a flowchart indicating a third example of inter predictionprocessing performed by inter predictor 126 included in encoder 100.

Compared to the processes illustrated in FIG. 14 , Step S116 is newlyincluded in, and Step S107 is deleted from the processes illustrated inFIG. 17 . In addition, Step S109 is changed to Step S109A.

When the merge mode or the FRUC mode, which are the modes of the methodnot involving encoding of a motion vector difference other than theaffine mode, is used (0 or 1 in S101), subsequent to Step S102 or StepS103, inter predictor 126 determines whether to apply a luminancecorrection process for a prediction image (S116). When the luminancecorrection process for a prediction image is determined not to beapplied (No in S116), inter predictor 126: determines whether a residualcoefficient having a nonzero value is present (S105); encodes a currentblock via the skip mode (S106) when the residual coefficient having anonzero value is not present (No in S105); and encodes a current blockvia the non-skip mode (S108) when the residual coefficient having anonzero value is present (Yes in S105).

In addition, inter predictor 126 does not encode a luminance correctionprocessing signal in the case of the skip mode, and encodes theluminance correction processing signal by setting the luminancecorrection processing signal to a value indicating non-application inthe case of the non-skip mode (S109A). In the case of the non-skip mode,inter predictor 126 always encodes the residual coefficient informationwithout encoding the residual coefficient presence signal (S110).

In addition, in the processes illustrated in FIG. 17 , when theluminance correction process for a prediction image is applied in themerge mode or the FRUC mode (Yes in S116), the processes equivalent tothe processes performed when the affine mode or the method involvingencoding of a motion vector difference is used are performed.

In the cases where: the method involving encoding of a motion vectordifference is used (3 in S101); the affine mode among the modes of themethod not involving encoding of a motion vector difference is used (2in S101); and the method not involving encoding of a motion vectordifference is used and the luminance correction process for a predictionimage is applied (Yes in S116), inter predictor 126 always encodes acurrent block via the non-skip mode (S112), and encodes the luminancecorrection processing signal (S113). In addition, inter predictor 126encodes a residual coefficient presence signal (S114), and encodesresidual coefficient information (S110) when a residual coefficienthaving a nonzero value is present (Yes in S115).

[Third Example of Processing Performed by Inter Predictor of Decoder]

FIG. 18 is a flowchart which indicates a third example of interprediction processing performed by inter predictor 218 included indecoder 200 which decodes a stream generated by encoder 100 anddescribed in FIG. 17 .

Compared to the processes illustrated in FIG. 15 , Step S216 is newlyincluded in, and Step S207 is deleted from the processes illustrated inFIG. 18 .

In the case where the merge mode or the FRUC mode is used (0 or 1 inS201), which are the modes of the method not involving encoding of amotion vector difference other than the affine mode, inter predictor218: decodes a current block via the skip mode (S206) when a signalindicating that the skip mode is to be used is decoded (Yes in S205);and decodes a current block via the non-skip mode (S208) in the othercase (No in S205).

In addition, inter predictor 218 does not decode a luminance correctionprocessing signal in the case of the skip mode, and decodes theluminance correction processing signal in the case of the non-skip mode(S209). When the decoded luminance correction processing signalindicates that the luminance correction process for a prediction imageis to be applied (Yes in S216), inter predictor 218 decodes a residualcoefficient presence signal (S214), and decodes residual coefficientinformation (S210) when a residual coefficient is present (Yes in S215).A residual coefficient is always present when (i) the mode is thenon-skip mode, and (ii) the luminance correction processing signalindicates that the luminance correction process for a prediction imageis not to be applied (No in S216), and thus inter predictor 218 alwaysdecodes a residual coefficient information without decoding the residualcoefficient presence signal (S210).

It should be noted that the operations of the case where the methodinvolving decoding of a motion vector difference is used and the casewhere the affine mode among the modes of the method not involvingencoding of the motion vector difference is used (2 or 3 in S201) areequivalent to the operation illustrated in FIG. 15 .

[Syntax Structure of Third Example]

FIG. 19 is a syntax table which indicates a third example of a syntaxstructure of the stream generated by encoder 100 and described in FIG.17 .

First, “skip_flag” specifies whether to use the skip mode or to use thenon-skip mode.

In the case where the skip mode is used, further, “fruc_mode” specifieswhether to use the FRUC mode. In the case where the FRUC mode is notused, “merge_idx” for specifying a surrounding block to be referred toin the merge mode is described.

In the case where the non-skip mode is used, “merge_flag” specifieswhether to use the method not involving encoding of a motion vectordifference or to use a method involving encoding of the motion vectordifference.

In the case where the method not involving encoding of a motion vectordifference is used, further, “fruc_mode” specifies whether to use theFRUC mode. In the case where the FRUC mode is not used, further,“affine_flag” specifies whether to use the affine mode. In the casewhere the affine mode is not used, “merge_idx” for specifying asurrounding block to be referred to in the merge mode is described.

In the case where the method involving encoding of a motion vectordifference is used, “MVD” that is information related to the motionvector difference is described.

It should be noted that, in any case, “lic_flag” which indicates whetherto apply a luminance correction process for a prediction image isdescribed. In addition, in the case where: the method not involvingencoding of a motion vector difference is not used; the affine modeamong the modes of the method not involving encoding of a motion vectordifference is used; or it is described that the luminance correctionprocess for a prediction image is to be applied, “root_cbf” whichindicates whether a residual coefficient having a nonzero value ispresent is described. In the case where it is indicated that theresidual coefficient having a nonzero value is present, “residual” thatis residual coefficient information is described.

[Advantageous Effect of Third Example]

According to the third example, when the skip mode is selected, only“fruc_mode” and “merge_idx” need to be described. Thus, when, forexample, encoding is performed using a large number of skip modes undera low-rate encoding condition, etc., the number of items of syntaxdescribed in a stream is less than that of the second example describedwith reference to FIG. 14 to FIG. 16 . Accordingly, there is apossibility that coding efficiency can be further improved. Inparticular, in the affine mode or the luminance correction process for aprediction image, since it is highly likely that a residual coefficienthaving a nonzero value is generated, the advantageous effect ofreduction in the code amount resulting from not describing “affine_flag”and “lic_flag” in the skip mode is likely to be great. In addition,there is a possibility that the circuitry for controlling syntax can bemore simplified.

It should be noted that not all of the constituent elements described inthe third example are always necessary, and encoder 100 or decoder 200may be provided with only part of the constituent elements described inthe third example.

For example, although four modes respectively associated with 0 to 3 aredescribed as examples of the inter prediction mode information in FIG.17 and FIG. 18 , those numerical numbers and modes are merely examples,and thus other numerical numbers and modes may be used. In particular,although the example of using three modes, namely, the merge mode, theFRUC mode, and the affine mode, in the method not involving encoding ofa motion vector difference is described, at least two modes among theabove-described modes are sufficient to be used. In addition, a modeother than the above-described modes may be used. Furthermore,processing equivalent to the processing described regarding the affinemode may be applied to a mode different from the affine mode.

In addition, the syntax structure described with reference to FIG. 19 ismerely one example, and part of the syntax in FIG. 19 may be replacedwith other syntax, or deletion or addition may be carried out.

Compared to the processing described in the second example, theprocessing related to the luminance correction process for a predictionimage is newly included in the third example. However, the processingrelated to the luminance correction process for a prediction image maybe added to the processing of the first example described with referenceto FIG. 11 to FIG. 13 . In this case, although “affine_flag” isdescribed as well when the skip mode is selected, it is not necessary todescribe “lic_flag”. Accordingly, there is a possibility that codingefficiency is further improved compared to the structure of the firstexample described with reference to FIG. 11 to FIG. 13 .

[LIC Processing]

The LIC processing (luminance correction process) has been describedabove with reference FIG. 9D. The following describes the details of theLIC processing.

First, inter predictor 126 derives a motion vector for obtaining areference image corresponding to a current block to be encoded, from areference picture that is an encoded picture.

Next, inter predictor 126 extracts information indicating how theluminance value changed between the reference picture and the currentpicture to calculate a luminance correction parameter, by A residualcoefficient is always present when (i) the mode is the non-skip mode,and (ii) using the luminance pixel values for the encoded leftneighboring reference region and the encoded upper neighboring referenceregion, and the luminance pixel value in the corresponding position inthe reference picture specified by the motion vector. For example, it isassumed that the luminance pixel value of one pixel in a surroundingreference region in the current picture to be encoded is p0, and theluminance pixel value of a pixel in a surrounding reference region in areference picture at the position corresponding to the one pixel is p1.Inter predictor 126 calculates, as luminance correction parameters,coefficients A and B which optimize A×p1+B=p0, for a plurality of pixelsin the surrounding reference region.

Next, inter predictor 126 generates a prediction image for the currentblock by performing a luminance correction process by using theluminance correction parameter on the reference image in the referencepicture specified by the motion vector. For example, it is assumed thatthe luminance pixel value in the reference image is p2, and theluminance pixel value in the prediction image after the luminancecorrection process is p3. Inter predictor 126 generates the predictionimage after the luminance correction process, by calculating A×p2+B=p3for each pixel in the reference image.

It should be noted that the shape of the surrounding reference regionillustrated in FIG. 9D is merely one example, and thus the surroundingreference region may have a different shape. In addition, part of thesurrounding reference region illustrated in FIG. 9D may be used.Moreover, the surrounding reference region is not limited to the regionadjacent to the current block, and may be a region not adjacent to thecurrent block. Furthermore, although the surrounding reference region inthe reference picture is the region specified by the motion vector ofthe current picture, based on the surrounding reference region in thecurrent picture in the example illustrated in FIG. 9D, the surroundingreference region in the reference picture may be a region specified byanother motion vector. For example, such another motion vector may be amotion vector of the surrounding reference region in the currentpicture.

It should be noted that, although an operation performed by encoder 100has been described here, the same applies to an operation performed bydecoder 200.

[Others]

The mode referred to as a skip mode in the present disclosure may bereferred to as a different name. The skip mode according to the presentdisclosure is a mode specified by a skip flag (“skip_flag” in syntax),for example.

The following provides some examples of the condition for applying askip mode, based on the descriptions in each of the aspects of thepresent disclosure.

In the first example of the inter prediction processing performed byinter predictor 126 included in encoder 100 according to the presentdisclosure, in the method not involving encoding of a signal related toa motion vector difference, the mode for generating a stream having asyntax structure of not encoding a signal related to a residualcoefficient is referred to as a skip mode, and other modes are referredto as non-skip modes.

In the first example of the inter prediction processing performed byinter predictor 218 included in decoder 200 according to the presentdisclosure, in the method not involving decoding of a signal related toa motion vector difference, the mode for decoding a stream having asyntax structure in which a signal related to a residual coefficient isnot encoded is referred to as a skip mode, and other modes are referredto as non-skip modes.

In the second example of the inter prediction processing performed byinter predictor 126 included in encoder 100 according to the presentdisclosure, in the method involving deriving a motion vector via themerge mode or the FRUC mode, the mode for generating a stream having asyntax structure of not encoding a signal related to a residualcoefficient is referred to as a skip mode, and other modes are referredto as non-skip modes.

In the second example of the inter prediction processing performed byinter predictor 218 included in decoder 200 according to the presentdisclosure, in the method involving deriving a motion vector via themerge mode or the FRUC mode, the mode for decoding a stream having asyntax structure in which a signal related to a residual coefficient isnot encoded is referred to as a skip mode, and other modes are referredto as a non-skip mode.

In the third example of the inter prediction processing performed byinter predictor 126 included in encoder 100 according to the presentdisclosure, in the method involving deriving a motion vector via themerge mode or the FRUC mode, the mode in which: the luminance correctionprocess for a prediction image is not applied; and a stream having asyntax structure in which a signal related to a residual coefficient isnot encoded is generated is referred to as a skip mode, and other modesare referred to as non-skip modes

In the third example of the inter prediction processing performed byinter predictor 218 included in decoder 200 according to the presentdisclosure, in the method involving deriving a motion vector via themerge mode or the FRUC mode, the mode in which: the luminance correctionprocess for a prediction image is not applied; and a stream having asyntax structure in which a signal related to a residual coefficient isnot encoded is decoded is referred to as a skip mode, and other modesare referred to as non-skip modes.

The signal related to a residual coefficient described above is a signalindicating a residual coefficient, for example.

It should be noted that the above-described condition for performingencoding via the skip mode is merely one example, and thus encoder 100or decoder 200 may perform encoding or decoding, by determining whetherto apply the skip mode based on a condition other than above-describedcondition.

For example, the method for deriving a motion vector, which is one ofthe determination criteria for the condition for performing encoding viathe skip mode, may be limited only to the case of a single method suchas the merge mode, or may be a combination of the merge mode and theaffine mode. In addition, the method for deriving a motion vectorwithout encoding a signal related to a motion vector difference otherthan the merge mode, the FRUC mode, and the affine mode described asexamples in the present disclosure may be the subject for applying theskip mode.

In part of the aspects among the examples of the inter predictionprocessing performed by inter predictor of encoder 100 or decoder 200described in present disclosure, encoder 100 or decoder 200, in one orsome of the modes for deriving a motion vector without encoding a signalrelated to a motion vector difference, performs encoding withoutapplying the mode specified by a skip flag even when the signal relatedto a residual coefficient is not encoded. According to thisconfiguration, it is possible to perform encoding without including, insyntax for the mode specified by a skip flag, information or a flagwhich indicates whether to use the mode for deriving a motion vectorwhich is not the subject to be applied with the mode specified by theskip flag. As a result, as to a low-probability condition, for example,in the case where the coding efficiency rather decreases by including,in the syntax for the mode specified by the skip flag, information or aflag which indicates whether the low-probability condition is satisfied,there is a possibility that the coding efficiency can be improved.

In part of the aspects among the examples of the inter predictionprocessing performed by inter predictor of encoder 100 or decoder 200described in present disclosure, encoder 100 or decoder 200, in themethod not involving encoding of a signal related to a motion vectordifference, performs encoding without applying the mode specified by askip flag if a specific condition is satisfied, even when the signalrelated to a residual coefficient is not encoded. The specific conditionis, for example, the case where luminance correction processing isapplied for a prediction image. According to this configuration, it ispossible to perform encoding without including, in syntax for the modespecified by a skip flag, information or a flag which indicates whetherthe condition that the mode specified by a skip flag is not applied issatisfied. As a result, as to a low-probability condition, for example,in the case where the coding efficiency rather decreases by including,in the syntax for the mode specified by the skip flag, information or aflag which indicates whether the low-probability condition is satisfied,there is a possibility that the coding efficiency can be improved.

[Implementation Example of Encoder]

FIG. 20 is a block diagram illustrating an implementation example ofencoder 100 according to Embodiment 1. Encoder 100 includes circuitry160 and memory 162. For example, the constituent elements included inencoder 100 illustrated in FIG. 1 are implemented by circuitry 160 andmemory 162 illustrated in FIG. 20 .

Circuitry 160 is a circuit which performs information processing, and iscapable of accessing memory 162. For example, circuitry 160 is adedicated or general purpose electronic circuit which encodes a video.Circuitry 160 may be a processor such as a CPU. Alternatively, circuitry160 may be an aggregation of a plurality of electronic circuits. Inaddition, for example, circuitry 160 may perform functions of aplurality of constituent elements other than constituent elements forstoring information, among a plurality of constituent elements ofencoder 100 illustrated in FIG. 1 , etc.

Memory 162 is dedicated or general purpose memory in which informationfor encoding a video by circuitry 160 is stored. Memory 162 may be anelectronic circuit, or may be connected to circuitry 160. Alternatively,memory 162 may be included in circuitry 160. Furthermore, memory 162 maybe an aggregation of a plurality of electronic circuits. In addition,memory 162 may be a magnetic disk, an optical disk, or the like, or maybe represented as storage, a recording medium, or the like. In addition,memory 162 may be non-volatile memory, or volatile memory.

For example, in memory 162, video to be encoded may be stored or abitstream corresponding to an encoded video may be stored. In addition,a program to be executed by circuitry 160 for encoding a video may bestored in memory 162.

In addition, for example, memory 162 may perform functions of aconstituent element for storing information, among a plurality ofconstituent elements of encoder 100 illustrated in FIG. 1 , etc.Specifically, memory 162 may perform the functions of block memory 118and frame memory 122 illustrated in FIG. 1 . More specifically,reconstructed blocks, reconstructed pictures, etc. may be stored inmemory 162.

It should be noted that, in encoder 100, not all the plurality ofconstituent elements illustrated in FIG. 1 , etc. may be mounted, or notall the plurality of processes described above may be performed. Part ofthe plurality of constituent elements illustrated in FIG. 1 , etc. maybe included in one or more other devices, and part of the plurality ofprocesses described above may be performed by one or more other devices.In encoder 100, part of the plurality of constituent elementsillustrated in FIG. 1 , etc. is mounted, and motion compensation isefficiently performed by means of part of the above-described processesbeing executed.

More specifically, encoder 100 selects a mode from among a plurality ofmodes each for deriving a motion vector, and derives a motion vector fora current block via the selected mode (S102, S103, S104, and S111 inFIG. 14 ). Next, encoder 100 performs inter prediction encoding on thecurrent block, using the motion vector derived, via one of a skip modeand a non-skip mode that is different from the skip mode (S106, S108, orS112 in FIG. 14 ). The plurality of modes include a plurality of firstmodes (for example, a merge mode, a FRUC mode, and an affine mode) ineach of which a motion vector of the current block is predicted based onan encoded block neighboring the current block without encodinginformation indicating the motion vector into a stream. When a secondmode included in the plurality of first modes is selected, encoder 100encodes the current block via the non-skip mode regardless of presenceor absence of a residual coefficient (S112 in FIG. 14 ).

According to this, encoder 100 is capable of increasing the codingefficiency. For example, since this eliminates the need for transmittinginformation indicating whether the second mode is used in the case wherethe skip mode is used, it is possible to improve the coding efficiency.

For example, when a third mode (for example, the merge mode or the FRUCmode) that is included in the plurality of the first modes and differentfrom the second mode, encoder 100 encodes the current block via thenon-skip mode (S108 in FIG. 14 ) when a residual coefficient is present(Yes in S105 in FIG. 14 ), and encodes the current block via the skipmode (S106 in FIG. 14 ) when a residual coefficient is not present (Noin S105 in FIG. 14 ).

For example, the second mode is a mode (affine mode) for predicting amotion vector corresponding to affine transformation, based on theencoded block neighboring the current block.

According to this, when a mode in which a non-skip mode is highly likelyto be selected as a result of a coefficient having a nonzero value beinggenerated is performed, the non-skip mode is used regardless of presenceor absence of a residual coefficient. Accordingly, it is possible toreduce the influence resulting from the skip mode being not selected.

For example, the third mode is one of a FRUC mode and a merge mode.

For example, encoder 100 further encodes information indicating presenceor absence of a residual coefficient when the current block is encodedvia the non-skip mode (S114 in FIG. 14 ).

For example, encoder 100, further, selects whether to perform, on thecurrent block, a luminance correction process by which a luminanceaverage value of a prediction image is corrected using a correctionvalue that is predicted from a luminance value of an encoded blockneighboring the current block (S116 in FIG. 17 ). Encoder 100 encodesthe current block via the non-skip mode regardless of the presence orabsence of a residual coefficient (S112 in FIG. 17 ) when the luminancecorrection process is performed on the current block (Yes in S116 inFIG. 17 ).

According to this, when the luminance correction process is performed,in which a non-skip mode is highly likely to be selected as a result ofa coefficient having a nonzero value being generated, the non-skip modeis used regardless of the presence or absence of a residual coefficient.Accordingly, it is possible to reduce the influence resulting from theskip mode being not selected.

For example, encoder 100, further, encodes information indicatingpresence or absence of a residual coefficient (S114 in FIG. 17 ) whenthe luminance correction process is performed on the current block (Yesin S116 in FIG. 17 ).

[Implementation Example of Decoder]

FIG. 21 is a block diagram illustrating an implementation example ofdecoder 200 according to Embodiment 1. Decoder 200 includes circuitry260 and memory 262. For example, the constituent elements included indecoder 200 illustrated in FIG. 10 are implemented by circuitry 260 andmemory 262 illustrated in FIG. 21 .

Circuitry 260 is a circuitry which performs information processing, andis capable of accessing memory 262. For example, circuitry 260 is adedicated or general purpose electronic circuit which decodes a video.Circuitry 260 may be a processor such as a CPU. Alternatively, circuitry260 may be an aggregation of a plurality of electronic circuits. Inaddition, for example, circuitry 260 may perform functions of aplurality of constituent elements other than constituent elements forstoring information, among a plurality of constituent elements ofdecoder 200 illustrated in FIG. 10 , etc.

Memory 262 is dedicated or general purpose memory in which informationfor decoding a video by circuitry 260 is stored. Memory 262 may be anelectronic circuit, or may be connected to circuitry 260. Alternatively,memory 262 may be included in circuitry 260. Furthermore, memory 262 maybe an aggregation of a plurality of electronic circuits. In addition,memory 262 may be a magnetic disk, an optical disk, or the like, or maybe represented as storage, a recording medium, or the like. In addition,memory 262 may be non-volatile memory, or volatile memory.

For example, a bitstream corresponding to an encoded video or a videocorresponding to a decoded bitstream may be stored in memory 262. Inaddition, a program to be executed by circuitry 260 for decoding a videomay be stored in memory 262.

In addition, for example, memory 262 may perform functions of aconstituent element for storing information, among a plurality ofconstituent elements of decoder 200 illustrated in FIG. 10 , etc. Morespecifically, memory 262 may perform the functions of block memory 210and frame memory 214 illustrated in FIG. 10 . More specifically,reconstructed blocks, reconstructed pictures, etc. may be stored inmemory 262.

It should be noted that, in decoder 200, not all the plurality ofconstituent elements illustrated in FIG. 10 , etc. may be mounted, ornot all the plurality of processes described above may be performed.Part of the plurality of constituent elements illustrated in FIG. 10 ,etc. may be included in one or more other devices, and part of theplurality of processes described above may be performed by one or moreother devices. In decoder 200, part of the plurality of constituentelements illustrated in FIG. 10 , etc. is mounted, and motioncompensation is efficiently performed by means of part of theabove-described processes being executed.

More specifically, decoder 200 selects a mode from among a plurality ofmodes each for deriving a motion vector, and derives a motion vector fora current block via the selected mode (S202, S203, S204, and S211 inFIG. 15 ). Next, decoder 200 performs inter prediction decoding on thecurrent block, using the motion vector derived, via one of a skip modeand a non-skip mode that is different from the skip mode (S206, S208, orS212 in FIG. 15 ). The plurality of modes include a plurality of firstmodes (for example, a merge mode, a FRUC mode, and an affine mode) ineach of which a motion vector of the current block is predicted based ona decoded block neighboring the current block without decodinginformation indicating the motion vector from a stream. When a secondmode included in the plurality of first modes is selected, decoder 200decodes the current block via the non-skip mode regardless of presenceor absence of a residual coefficient (S212 in FIG. 15 ).

According to this, decoder 200 is capable of improving the codingefficiency. For example, since this eliminates the need for transmittinginformation indicating whether the second mode is used in the case wherethe skip mode is used, it is possible to improve the coding efficiency.

For example, when a third mode (for example, the merge mode or the FRUCmode) is elected, which is included in the plurality of the first modesand different from the second mode, decoder 200 decodes the currentblock via the non-skip mode (S208 in FIG. 15 ) when a residualcoefficient is present (No in S205 in FIG. 15 ), and decodes the currentblock via the skip mode (S206 in FIG. 15 ) when a residual coefficientis not present (Yes in S205 in FIG. 15 ).

For example, the second mode is a mode (affine mode) in which motionvector prediction corresponding to affine transformation is performed,based on the decoded block neighboring the current block.

According to this, when a mode in which a non-skip mode is highly likelyto be selected as a result of a coefficient having a nonzero value beinggenerated is performed, the non-skip mode is used regardless of presenceor absence of a residual coefficient. Accordingly, it is possible toreduce the influence resulting from the skip mode being not selected.

For example, the third mode is one of a FRUC mode and a merge mode.

For example, decoder 200, further, decodes information indicatingpresence or absence of a residual coefficient when the current block isdecoded via the non-skip mode (S214 in FIG. 18 ).

For example, decoder 200, further, selects whether to perform, on thecurrent block, a luminance correction process by which a luminanceaverage value of a prediction image is corrected using a correctionvalue that is predicted from a luminance value of a decoded blockneighboring the current block (S216 in FIG. 18 ). Decoder 200 decodesthe current block via the non-skip mode regardless of the presence orabsence of a residual coefficient (S208 in FIG. 18 ) when the luminancecorrection process is performed on the current block (Yes in S216 inFIG. 18 ).

According to this, when the luminance correction process is performed,in which a non-skip mode is highly likely to be selected as a result ofa coefficient having a nonzero value being generated, the non-skip modeis used regardless of the presence or absence of a residual coefficient.Accordingly, it is possible to reduce the influence resulting from theskip mode being not selected.

For example, decoder 200, further, decodes information indicatingpresence or absence of a residual coefficient (S214) when the luminancecorrection process is performed on the current block (Yes in S216 inFIG. 18 ).

[Supplements]

Encoder 100 and decoder 200 according to the present embodiment may beused as an image encoder and an image decoder, respectively, or as avideo encoder and a video decoder, respectively. Alternatively, each ofencoder 100 and decoder 200 can be used as an inter prediction apparatus(inter-picture prediction apparatus).

In other words, encoder 100 and decoder 200 may correspond only to interpredictor (inter-picture predictor) 126 and inter predictor(inter-picture predictor) 218, respectively. In addition, otherconstituent elements such as transformer 106 and inverse transformer 206may be included in another apparatus.

It should be noted that, each of the constituent elements in the presentembodiment may be configured in the form of an exclusive hardwareproduct, or may be realized by executing a software program suitable forthe constituent element. Each of the constituent elements may berealized by means of a program executing unit, such as a CPU and aprocessor, reading and executing the software program recorded on arecording medium such as a hard disk or a semiconductor memory.

More specifically, each of encoder 100 and decoder 200 may includeprocessing circuitry and storage which is electrically connected to theprocessing circuitry and accessible from the processing circuitry. Forexample, the processing circuitry corresponds to circuitry 160 or 260,and the storage corresponds to memory 162 or 262.

The processing circuitry includes at least one of the exclusive hardwareand the program executing unit, and executes the processing using thestorage. In addition, when the processing circuitry includes the programexecuting unit, the storage stores a software program that is executedby the program executing unit.

Here, the software for implementing encoder 100, decoder 200, or thelike according to this embodiment includes programs as indicated below.

In addition, each of the constituent elements may be circuitry asdescribed above. The circuitries may be configured as a single circuitryas a whole or may be mutually different circuitries. In addition, eachof the constituent elements may be implemented as a general purposeprocessor or as a dedicated processor.

In addition, processes executed by a specific constituent element may beperformed by a different constituent element. In addition, the order inwhich processes are performed may be changed, or a plurality ofprocesses may be performed in parallel. In addition, an encoder/decodermay include encoder 100 and decoder 200.

Aspects of encoder 100 and decoder 200 have been described above basedon the embodiments. However, aspects of encoder 100 and decoder 200 arenot limited to the embodiments described above. The one or more aspectsof the present disclosure may encompass embodiments obtainable byadding, to the embodiments, various kinds of modifications that a personskilled in the art would arrive at and embodiments configurable bycombining constituent elements in different embodiments within the scopeof the aspects of encoder 100 and decoder 200.

This aspect may be implemented in combination with one or more of theother aspects according to the present disclosure. In addition, part ofthe processes in the flowcharts, part of the constituent elements of theapparatuses, and part of the syntax described in this aspect may beimplemented in combination with other aspects.

Embodiment 2

As described in each of the above embodiments, each functional block cantypically be realized as an MPU and memory, for example. Moreover,processes performed by each of the functional blocks are typicallyrealized by a program execution unit, such as a processor, reading andexecuting software (a program) recorded on a recording medium such asROM. The software may be distributed via, for example, downloading, andmay be recorded on a recording medium such as semiconductor memory anddistributed. Note that each functional block can, of course, also berealized as hardware (dedicated circuit).

Moreover, the processing described in each of the embodiments may berealized via integrated processing using a single apparatus (system),and, alternatively, may be realized via decentralized processing using aplurality of apparatuses. Moreover, the processor that executes theabove-described program may be a single processor or a plurality ofprocessors. In other words, integrated processing may be performed, and,alternatively, decentralized processing may be performed.

Embodiments of the present disclosure are not limited to the aboveexemplary embodiments; various modifications may be made to theexemplary embodiments, the results of which are also included within thescope of the embodiments of the present disclosure.

Next, application examples of the moving picture encoding method (imageencoding method) and the moving picture decoding method (image decodingmethod) described in each of the above embodiments and a system thatemploys the same will be described. The system is characterized asincluding an image encoder that employs the image encoding method, animage decoder that employs the image decoding method, and an imageencoder/decoder that includes both the image encoder and the imagedecoder. Other configurations included in the system may be modified ona case-by-case basis.

[Usage Examples]

FIG. 22 illustrates an overall configuration of content providing systemex100 for implementing a content distribution service. The area in whichthe communication service is provided is divided into cells of desiredsizes, and base stations ex106, ex107, ex108, ex109, and ex110, whichare fixed wireless stations, are located in respective cells.

In content providing system ex100, devices including computer ex111,gaming device ex112, camera ex113, home appliance ex114, and smartphoneex115 are connected to internet ex101 via internet service providerex102 or communications network ex104 and base stations ex106 throughex110. Content providing system ex100 may combine and connect anycombination of the above elements. The devices may be directly orindirectly connected together via a telephone network or near fieldcommunication rather than via base stations ex106 through ex110, whichare fixed wireless stations. Moreover, streaming server ex103 isconnected to devices including computer ex111, gaming device ex112,camera ex113, home appliance ex114, and smartphone ex115 via, forexample, internet ex101. Streaming server ex103 is also connected to,for example, a terminal in a hotspot in airplane ex117 via satelliteex116.

Note that instead of base stations ex106 through ex110, wireless accesspoints or hotspots may be used. Streaming server ex103 may be connectedto communications network ex104 directly instead of via internet ex101or internet service provider ex102, and may be connected to airplaneex117 directly instead of via satellite ex116.

Camera ex113 is a device capable of capturing still images and video,such as a digital camera. Smartphone ex115 is a smartphone device,cellular phone, or personal handyphone system (PHS) phone that canoperate under the mobile communications system standards of the typical2G, 3G, 3.9G, and 4G systems, as well as the next-generation 5G system.

Home appliance ex118 is, for example, a refrigerator or a deviceincluded in a home fuel cell cogeneration system.

In content providing system ex100, a terminal including an image and/orvideo capturing function is capable of, for example, live streaming byconnecting to streaming server ex103 via, for example, base stationex106. When live streaming, a terminal (e.g., computer ex111, gamingdevice ex112, camera ex113, home appliance ex114, smartphone ex115, orairplane ex117) performs the encoding processing described in the aboveembodiments on still-image or video content captured by a user via theterminal, multiplexes video data obtained via the encoding and audiodata obtained by encoding audio corresponding to the video, andtransmits the obtained data to streaming server ex103. In other words,the terminal functions as the image encoder according to one aspect ofthe present disclosure.

Streaming server ex103 streams transmitted content data to clients thatrequest the stream. Client examples include computer ex111, gamingdevice ex112, camera ex113, home appliance ex114, smartphone ex115, andterminals inside airplane ex117, which are capable of decoding theabove-described encoded data. Devices that receive the streamed datadecode and reproduce the received data. In other words, the devices eachfunction as the image decoder according to one aspect of the presentdisclosure.

[Decentralized Processing]

Streaming server ex103 may be realized as a plurality of servers orcomputers between which tasks such as the processing, recording, andstreaming of data are divided. For example, streaming server ex103 maybe realized as a content delivery network (CDN) that streams content viaa network connecting multiple edge servers located throughout the world.In a CDN, an edge server physically near the client is dynamicallyassigned to the client. Content is cached and streamed to the edgeserver to reduce load times. In the event of, for example, some kind ofan error or a change in connectivity due to, for example, a spike intraffic, it is possible to stream data stably at high speeds since it ispossible to avoid affected parts of the network by, for example,dividing the processing between a plurality of edge servers or switchingthe streaming duties to a different edge server, and continuingstreaming.

Decentralization is not limited to just the division of processing forstreaming; the encoding of the captured data may be divided between andperformed by the terminals, on the server side, or both. In one example,in typical encoding, the processing is performed in two loops. The firstloop is for detecting how complicated the image is on a frame-by-frameor scene-by-scene basis, or detecting the encoding load. The second loopis for processing that maintains image quality and improves encodingefficiency. For example, it is possible to reduce the processing load ofthe terminals and improve the quality and encoding efficiency of thecontent by having the terminals perform the first loop of the encodingand having the server side that received the content perform the secondloop of the encoding. In such a case, upon receipt of a decodingrequest, it is possible for the encoded data resulting from the firstloop performed by one terminal to be received and reproduced on anotherterminal in approximately real time. This makes it possible to realizesmooth, real-time streaming.

In another example, camera ex113 or the like extracts a feature amountfrom an image, compresses data related to the feature amount asmetadata, and transmits the compressed metadata to a server. Forexample, the server determines the significance of an object based onthe feature amount and changes the quantization accuracy accordingly toperform compression suitable for the meaning of the image. Featureamount data is particularly effective in improving the precision andefficiency of motion vector prediction during the second compressionpass performed by the server. Moreover, encoding that has a relativelylow processing load, such as variable length coding (VLC), may behandled by the terminal, and encoding that has a relatively highprocessing load, such as context-adaptive binary arithmetic coding(CABAC), may be handled by the server.

In yet another example, there are instances in which a plurality ofvideos of approximately the same scene are captured by a plurality ofterminals in, for example, a stadium, shopping mall, or factory. In sucha case, for example, the encoding may be decentralized by dividingprocessing tasks between the plurality of terminals that captured thevideos and, if necessary, other terminals that did not capture thevideos and the server, on a per-unit basis. The units may be, forexample, groups of pictures (GOP), pictures, or tiles resulting fromdividing a picture. This makes it possible to reduce load times andachieve streaming that is closer to real-time.

Moreover, since the videos are of approximately the same scene,management and/or instruction may be carried out by the server so thatthe videos captured by the terminals can be cross-referenced. Moreover,the server may receive encoded data from the terminals, change referencerelationship between items of data or correct or replace picturesthemselves, and then perform the encoding. This makes it possible togenerate a stream with increased quality and efficiency for theindividual items of data.

Moreover, the server may stream video data after performing transcodingto convert the encoding format of the video data. For example, theserver may convert the encoding format from MPEG to VP, and may convertH.264 to H.265.

In this way, encoding can be performed by a terminal or one or moreservers. Accordingly, although the device that performs the encoding isreferred to as a “server” or “terminal” in the following description,some or all of the processes performed by the server may be performed bythe terminal, and likewise some or all of the processes performed by theterminal may be performed by the server. This also applies to decodingprocesses.

[3D, Multi-Angle]

In recent years, usage of images or videos combined from images orvideos of different scenes concurrently captured or the same scenecaptured from different angles by a plurality of terminals such ascamera ex113 and/or smartphone ex115 has increased. Videos captured bythe terminals are combined based on, for example, theseparately-obtained relative positional relationship between theterminals, or regions in a video having matching feature points.

In addition to the encoding of two-dimensional moving pictures, theserver may encode a still image based on scene analysis of a movingpicture either automatically or at a point in time specified by theuser, and transmit the encoded still image to a reception terminal.Furthermore, when the server can obtain the relative positionalrelationship between the video capturing terminals, in addition totwo-dimensional moving pictures, the server can generatethree-dimensional geometry of a scene based on video of the same scenecaptured from different angles. Note that the server may separatelyencode three-dimensional data generated from, for example, a pointcloud, and may, based on a result of recognizing or tracking a person orobject using three-dimensional data, select or reconstruct and generatea video to be transmitted to a reception terminal from videos capturedby a plurality of terminals.

This allows the user to enjoy a scene by freely selecting videoscorresponding to the video capturing terminals, and allows the user toenjoy the content obtained by extracting, from three-dimensional datareconstructed from a plurality of images or videos, a video from aselected viewpoint. Furthermore, similar to with video, sound may berecorded from relatively different angles, and the server may multiplex,with the video, audio from a specific angle or space in accordance withthe video, and transmit the result.

In recent years, content that is a composite of the real world and avirtual world, such as virtual reality (VR) and augmented reality (AR)content, has also become popular. In the case of VR images, the servermay create images from the viewpoints of both the left and right eyesand perform encoding that tolerates reference between the two viewpointimages, such as multi-view coding (MVC), and, alternatively, may encodethe images as separate streams without referencing. When the images aredecoded as separate streams, the streams may be synchronized whenreproduced so as to recreate a virtual three-dimensional space inaccordance with the viewpoint of the user.

In the case of AR images, the server superimposes virtual objectinformation existing in a virtual space onto camera informationrepresenting a real-world space, based on a three-dimensional positionor movement from the perspective of the user. The decoder may obtain orstore virtual object information and three-dimensional data, generatetwo-dimensional images based on movement from the perspective of theuser, and then generate superimposed data by seamlessly connecting theimages. Alternatively, the decoder may transmit, to the server, motionfrom the perspective of the user in addition to a request for virtualobject information, and the server may generate superimposed data basedon three-dimensional data stored in the server in accordance with thereceived motion, and encode and stream the generated superimposed datato the decoder. Note that superimposed data includes, in addition to RGBvalues, an a value indicating transparency, and the server sets the avalue for sections other than the object generated fromthree-dimensional data to, for example, 0, and may perform the encodingwhile those sections are transparent. Alternatively, the server may setthe background to a predetermined RGB value, such as a chroma key, andgenerate data in which areas other than the object are set as thebackground.

Decoding of similarly streamed data may be performed by the client(i.e., the terminals), on the server side, or divided therebetween. Inone example, one terminal may transmit a reception request to a server,the requested content may be received and decoded by another terminal,and a decoded signal may be transmitted to a device having a display. Itis possible to reproduce high image quality data by decentralizingprocessing and appropriately selecting content regardless of theprocessing ability of the communications terminal itself. In yet anotherexample, while a TV, for example, is receiving image data that is largein size, a region of a picture, such as a tile obtained by dividing thepicture, may be decoded and displayed on a personal terminal orterminals of a viewer or viewers of the TV. This makes it possible forthe viewers to share a big-picture view as well as for each viewer tocheck his or her assigned area or inspect a region in further detail upclose.

In the future, both indoors and outdoors, in situations in which aplurality of wireless connections are possible over near, mid, and fardistances, it is expected to be able to seamlessly receive content evenwhen switching to data appropriate for the current connection, using astreaming system standard such as MPEG-DASH. With this, the user canswitch between data in real time while freely selecting a decoder ordisplay apparatus including not only his or her own terminal, but also,for example, displays disposed indoors or outdoors. Moreover, based on,for example, information on the position of the user, decoding can beperformed while switching which terminal handles decoding and whichterminal handles the displaying of content. This makes it possible to,while in route to a destination, display, on the wall of a nearbybuilding in which a device capable of displaying content is embedded oron part of the ground, map information while on the move. Moreover, itis also possible to switch the bit rate of the received data based onthe accessibility to the encoded data on a network, such as when encodeddata is cached on a server quickly accessible from the receptionterminal or when encoded data is copied to an edge server in a contentdelivery service.

[Scalable Encoding]

The switching of content will be described with reference to a scalablestream, illustrated in FIG. 23 , that is compression coded viaimplementation of the moving picture encoding method described in theabove embodiments. The server may have a configuration in which contentis switched while making use of the temporal and/or spatial scalabilityof a stream, which is achieved by division into and encoding of layers,as illustrated in FIG. 23 . Note that there may be a plurality ofindividual streams that are of the same content but different quality.In other words, by determining which layer to decode up to based oninternal factors, such as the processing ability on the decoder side,and external factors, such as communication bandwidth, the decoder sidecan freely switch between low resolution content and high resolutioncontent while decoding. For example, in a case in which the user wantsto continue watching, at home on a device such as a TV connected to theinternet, a video that he or she had been previously watching onsmartphone ex115 while on the move, the device can simply decode thesame stream up to a different layer, which reduces server side load.

Furthermore, in addition to the configuration described above in whichscalability is achieved as a result of the pictures being encoded perlayer and the enhancement layer is above the base layer, the enhancementlayer may include metadata based on, for example, statisticalinformation on the image, and the decoder side may generate high imagequality content by performing super-resolution imaging on a picture inthe base layer based on the metadata. Super-resolution imaging may beimproving the SN ratio while maintaining resolution and/or increasingresolution. Metadata includes information for identifying a linear or anon-linear filter coefficient used in super-resolution processing, orinformation identifying a parameter value in filter processing, machinelearning, or least squares method used in super-resolution processing.

Alternatively, a configuration in which a picture is divided into, forexample, tiles in accordance with the meaning of, for example, an objectin the image, and on the decoder side, only a partial region is decodedby selecting a tile to decode, is also acceptable. Moreover, by storingan attribute about the object (person, car, ball, etc.) and a positionof the object in the video (coordinates in identical images) asmetadata, the decoder side can identify the position of a desired objectbased on the metadata and determine which tile or tiles include thatobject. For example, as illustrated in FIG. 24 , metadata is storedusing a data storage structure different from pixel data such as an SEImessage in HEVC. This metadata indicates, for example, the position,size, or color of the main object.

Moreover, metadata may be stored in units of a plurality of pictures,such as stream, sequence, or random access units. With this, the decoderside can obtain, for example, the time at which a specific personappears in the video, and by fitting that with picture unit information,can identify a picture in which the object is present and the positionof the object in the picture.

Moreover, metadata may be stored in units of a plurality of pictures,such as stream, sequence, or random access units. With this, the decoderside can obtain, for example, the time at which a specific personappears in the video, and by fitting that with picture unit information,can identify a picture in which the object is present and the positionof the object in the picture.

When an image link is selected by the user, the display apparatusdecodes giving the highest priority to the base layer. Note that ifthere is information in the HTML code of the web page indicating thatthe content is scalable, the display apparatus may decode up to theenhancement layer. Moreover, in order to guarantee real timereproduction, before a selection is made or when the bandwidth isseverely limited, the display apparatus can reduce delay between thepoint in time at which the leading picture is decoded and the point intime at which the decoded picture is displayed (that is, the delaybetween the start of the decoding of the content to the displaying ofthe content) by decoding and displaying only forward reference pictures(I picture, P picture, forward reference B picture). Moreover, thedisplay apparatus may purposely ignore the reference relationshipbetween pictures and coarsely decode all B and P pictures as forwardreference pictures, and then perform normal decoding as the number ofpictures received over time increases.

[Autonomous Driving]

When transmitting and receiving still image or video data such two- orthree-dimensional map information for autonomous driving or assisteddriving of an automobile, the reception terminal may receive, inaddition to image data belonging to one or more layers, information on,for example, the weather or road construction as metadata, and associatethe metadata with the image data upon decoding. Note that metadata maybe assigned per layer and, alternatively, may simply be multiplexed withthe image data.

In such a case, since the automobile, drone, airplane, etc., includingthe reception terminal is mobile, the reception terminal can seamlesslyreceive and decode while switching between base stations among basestations ex106 through ex110 by transmitting information indicating theposition of the reception terminal upon reception request. Moreover, inaccordance with the selection made by the user, the situation of theuser, or the bandwidth of the connection, the reception terminal candynamically select to what extent the metadata is received or to whatextent the map information, for example, is updated.

With this, in content providing system ex100, the client can receive,decode, and reproduce, in real time, encoded information transmitted bythe user.

[Streaming of Individual Content]

In content providing system ex100, in addition to high image quality,long content distributed by a video distribution entity, unicast ormulticast streaming of low image quality, short content from anindividual is also possible. Moreover, such content from individuals islikely to further increase in popularity. The server may first performediting processing on the content before the encoding processing inorder to refine the individual content. This may be achieved with, forexample, the following configuration.

In real-time while capturing video or image content or after the contenthas been captured and accumulated, the server performs recognitionprocessing based on the raw or encoded data, such as capture errorprocessing, scene search processing, meaning analysis, and/or objectdetection processing. Then, based on the result of the recognitionprocessing, the server-either when prompted or automatically-edits thecontent, examples of which include: correction such as focus and/ormotion blur correction; removing low-priority scenes such as scenes thatare low in brightness compared to other pictures or out of focus; objectedge adjustment; and color tone adjustment. The server encodes theedited data based on the result of the editing. It is known thatexcessively long videos tend to receive fewer views. Accordingly, inorder to keep the content within a specific length that scales with thelength of the original video, the server may, in addition to thelow-priority scenes described above, automatically clip out scenes withlow movement based on an image processing result. Alternatively, theserver may generate and encode a video digest based on a result of ananalysis of the meaning of a scene.

Note that there are instances in which individual content may includecontent that infringes a copyright, moral right, portrait rights, etc.Such an instance may lead to an unfavorable situation for the creator,such as when content is shared beyond the scope intended by the creator.Accordingly, before encoding, the server may, for example, edit imagesso as to blur faces of people in the periphery of the screen or blur theinside of a house, for example. Moreover, the server may be configuredto recognize the faces of people other than a registered person inimages to be encoded, and when such faces appear in an image, forexample, apply a mosaic filter to the face of the person. Alternatively,as pre- or post-processing for encoding, the user may specify, forcopyright reasons, a region of an image including a person or a regionof the background be processed, and the server may process the specifiedregion by, for example, replacing the region with a different image orblurring the region. If the region includes a person, the person may betracked in the moving picture, and the head region may be replaced withanother image as the person moves.

Moreover, since there is a demand for real-time viewing of contentproduced by individuals, which tends to be small in data size, thedecoder first receives the base layer as the highest priority andperforms decoding and reproduction, although this may differ dependingon bandwidth. When the content is reproduced two or more times, such aswhen the decoder receives the enhancement layer during decoding andreproduction of the base layer and loops the reproduction, the decodermay reproduce a high image quality video including the enhancementlayer. If the stream is encoded using such scalable encoding, the videomay be low quality when in an unselected state or at the start of thevideo, but it can offer an experience in which the image quality of thestream progressively increases in an intelligent manner. This is notlimited to just scalable encoding; the same experience can be offered byconfiguring a single stream from a low quality stream reproduced for thefirst time and a second stream encoded using the first stream as areference.

[Other Usage Examples]

The encoding and decoding may be performed by LSI ex500, which istypically included in each terminal. LSI ex500 may be configured of asingle chip or a plurality of chips. Software for encoding and decodingmoving pictures may be integrated into some type of a recording medium(such as a CD-ROM, a flexible disk, or a hard disk) that is readable by,for example, computer ex111, and the encoding and decoding may beperformed using the software. Furthermore, when smartphone ex115 isequipped with a camera, the video data obtained by the camera may betransmitted. In this case, the video data is coded by LSI ex500 includedin smartphone ex115.

Note that LSI ex500 may be configured to download and activate anapplication. In such a case, the terminal first determines whether it iscompatible with the scheme used to encode the content or whether it iscapable of executing a specific service. When the terminal is notcompatible with the encoding scheme of the content or when the terminalis not capable of executing a specific service, the terminal firstdownloads a codec or application software then obtains and reproducesthe content.

Aside from the example of content providing system ex100 that usesinternet ex101, at least the moving picture encoder (image encoder) orthe moving picture decoder (image decoder) described in the aboveembodiments may be implemented in a digital broadcasting system. Thesame encoding processing and decoding processing may be applied totransmit and receive broadcast radio waves superimposed with multiplexedaudio and video data using, for example, a satellite, even though thisis geared toward multicast whereas unicast is easier with contentproviding system ex100.

[Hardware Configuration]

FIG. 27 illustrates smartphone ex115. FIG. 28 illustrates aconfiguration example of smartphone ex115. Smartphone ex115 includesantenna ex450 for transmitting and receiving radio waves to and frombase station ex110, camera ex465 capable of capturing video and stillimages, and display ex458 that displays decoded data, such as videocaptured by camera ex465 and video received by antenna ex450. Smartphoneex115 further includes user interface ex466 such as a touch panel, audiooutput unit ex457 such as a speaker for outputting speech or otheraudio, audio input unit ex456 such as a microphone for audio input,memory ex467 capable of storing decoded data such as captured video orstill images, recorded audio, received video or still images, and mail,as well as decoded data, and slot ex464 which is an interface for SIMex468 for authorizing access to a network and various data. Note thatexternal memory may be used instead of memory ex467.

Moreover, main controller ex460 which comprehensively controls displayex458 and user interface ex466, power supply circuit ex461, userinterface input controller ex462, video signal processor ex455, camerainterface ex463, display controller ex459, modulator/demodulator ex452,multiplexer/demultiplexer ex453, audio signal processor ex454, slotex464, and memory ex467 are connected via bus ex470.

When the user turns the power button of power supply circuit ex461 on,smartphone ex115 is powered on into an operable state by each componentbeing supplied with power from a battery pack.

Smartphone ex115 performs processing for, for example, calling and datatransmission, based on control performed by main controller ex460, whichincludes a CPU, ROM, and RAM. When making calls, an audio signalrecorded by audio input unit ex456 is converted into a digital audiosignal by audio signal processor ex454, and this is applied with spreadspectrum processing by modulator/demodulator ex452 and digital-analogconversion and frequency conversion processing by transmitter/receiverex451, and then transmitted via antenna ex450. The received data isamplified, frequency converted, and analog-digital converted, inversespread spectrum processed by modulator/demodulator ex452, converted intoan analog audio signal by audio signal processor ex454, and then outputfrom audio output unit ex457. In data transmission mode, text,still-image, or video data is transmitted by main controller ex460 viauser interface input controller ex462 as a result of operation of, forexample, user interface ex466 of the main body, and similar transmissionand reception processing is performed. In data transmission mode, whensending a video, still image, or video and audio, video signal processorex455 compression encodes, via the moving picture encoding methoddescribed in the above embodiments, a video signal stored in memoryex467 or a video signal input from camera ex465, and transmits theencoded video data to multiplexer/demultiplexer ex453. Moreover, audiosignal processor ex454 encodes an audio signal recorded by audio inputunit ex456 while camera ex465 is capturing, for example, a video orstill image, and transmits the encoded audio data tomultiplexer/demultiplexer ex453. Multiplexer/demultiplexer ex453multiplexes the encoded video data and encoded audio data using apredetermined scheme, modulates and converts the data usingmodulator/demodulator (modulator/demodulator circuit) ex452 andtransmitter/receiver ex451, and transmits the result via antenna ex450.

When video appended in an email or a chat, or a video linked from a webpage, for example, is received, in order to decode the multiplexed datareceived via antenna ex450, multiplexer/demultiplexer ex453demultiplexes the multiplexed data to divide the multiplexed data into abitstream of video data and a bitstream of audio data, supplies theencoded video data to video signal processor ex455 via synchronous busex470, and supplies the encoded audio data to audio signal processorex454 via synchronous bus ex470. Video signal processor ex455 decodesthe video signal using a moving picture decoding method corresponding tothe moving picture encoding method described in the above embodiments,and video or a still image included in the linked moving picture file isdisplayed on display ex458 via display controller ex459. Moreover, audiosignal processor ex454 decodes the audio signal and outputs audio fromaudio output unit ex457. Note that since real-time streaming is becomingmore and more popular, there are instances in which reproduction of theaudio may be socially inappropriate depending on the user's environment.Accordingly, as an initial value, a configuration in which only videodata is reproduced, i.e., the audio signal is not reproduced, ispreferable. Audio may be synchronized and reproduced only when an input,such as when the user clicks video data, is received.

Although smartphone ex115 was used in the above example, threeimplementations are conceivable: a transceiver terminal including bothan encoder and a decoder; a transmitter terminal including only anencoder; and a receiver terminal including only a decoder. Further, inthe description of the digital broadcasting system, an example is givenin which multiplexed data obtained as a result of video data beingmultiplexed with, for example, audio data, is received or transmitted,but the multiplexed data may be video data multiplexed with data otherthan audio data, such as text data related to the video. Moreover, thevideo data itself rather than multiplexed data maybe received ortransmitted.

Although main controller ex460 including a CPU is described ascontrolling the encoding or decoding processes, terminals often includeGPUs. Accordingly, a configuration is acceptable in which a large areais processed at once by making use of the performance ability of the GPUvia memory shared by the CPU and GPU or memory including an address thatis managed so as to allow common usage by the CPU and GPU. This makes itpossible to shorten encoding time, maintain the real-time nature of thestream, and reduce delay. In particular, processing relating to motionestimation, deblocking filtering, sample adaptive offset (SAO), andtransformation/quantization can be effectively carried out by the GPUinstead of the CPU in units of, for example pictures, all at once.

This aspect may be implemented in combination with one or more of theother aspects according to the present disclosure. In addition, part ofthe processes in the flowcharts, part of the constituent elements of theapparatuses, and part of the syntax described in this aspect may beimplemented in combination with other aspects.

Although only some exemplary embodiments of the present disclosure havebeen described in detail above, those skilled in the art will readilyappreciate that many modifications are possible in the exemplaryembodiments without materially departing from the novel teachings andadvantages of the present disclosure. Accordingly, all suchmodifications are intended to be included within the scope of thepresent disclosure.

INDUSTRIAL APPLICABILITY

The present disclosure is applicable to, for example, televisionreceivers, digital video recorders, car navigation systems, mobilephones, digital cameras, digital video cameras, teleconference systems,electronic mirrors, etc.

1-16. (canceled)
 17. An encoder, comprising: circuitry; and memory,wherein using the memory, the circuitry: selects a mode from among aplurality of modes each for deriving a motion vector, and derives amotion vector for a current block via the mode selected; and performsinter prediction encoding on the current block, using the motion vectorderived, via one of a skip mode and a non-skip mode that is differentfrom the skip mode, the plurality of modes include a plurality of firstmodes each for predicting the motion vector for the current block basedon an encoded block neighboring the current block without encodinginformation indicating a motion vector difference into a stream, in theskip mode, out of information indicating whether a second mode includedin the plurality of first modes is used and information indicatingwhether a third mode that is included in the plurality of first modesand different from the second mode is used, only the informationindicating whether the third mode is used is encoded into the stream, inthe non-skip mode, the information indicating whether the second mode isused and the information indicating whether the third mode is used areencoded into the stream, and when the second mode is used, the currentblock is encoded via the non-skip mode regardless of presence or absenceof a residual coefficient.
 18. A decoder, comprising: circuitry; andmemory, wherein using the memory, the circuitry: selects a mode fromamong a plurality of modes each for deriving a motion vector, andderives a motion vector for a current block via the mode selected; andperforms inter prediction decoding on the current block, using themotion vector derived, via one of a skip mode and a non-skip mode thatis different from the skip mode, the plurality of modes include aplurality of first modes each for predicting the motion vector for thecurrent block based on a decoded block neighboring the current blockwithout decoding information indicating a motion vector difference froma stream, in the skip mode, out of information indicating whether asecond mode included in the plurality of first modes is used andinformation indicating whether a third mode that is included in theplurality of first modes and different from the second mode is used,only the information indicating whether the third mode is used isdecoded from the stream, in the non-skip mode, the informationindicating whether the second mode is used and the informationindicating whether the third mode is used are decoded from the stream,and when the second mode is used, the current block is decoded via thenon-skip mode regardless of presence or absence of a residualcoefficient.
 19. An encoding method, comprising: selecting a mode fromamong a plurality of modes each for deriving a motion vector, andderiving a motion vector for a current block via the mode selected; andperforming inter prediction encoding on the current block, using themotion vector derived, via one of a skip mode and a non-skip mode thatis different from the skip mode, wherein the plurality of modes includea plurality of first modes each for predicting the motion vector for thecurrent block based on an encoded block neighboring the current blockwithout encoding information indicating a motion vector difference intoa stream, in the skip mode, out of information indicating whether asecond mode included in the plurality of first modes is used andinformation indicating whether a third mode that is included in theplurality of first modes and different from the second mode is used,only the information indicating whether the third mode is used isencoded into the stream, in the non-skip mode, the informationindicating whether the second mode is used and the informationindicating whether the third mode is used are encoded into the stream,and when the second mode is used, the current block is encoded via thenon-skip mode regardless of presence or absence of a residualcoefficient.
 20. A decoding method, comprising: selecting a mode fromamong a plurality of modes each for deriving a motion vector, andderiving a motion vector for a current block via the mode selected; andperforming inter prediction decoding on the current block, using themotion vector derived, via one of a skip mode and a non-skip mode thatis different from the skip mode, wherein the plurality of modes includea plurality of first modes each for predicting the motion vector for thecurrent block based on a decoded block neighboring the current blockwithout decoding information indicating a motion vector difference froma stream, in the skip mode, out of information indicating whether asecond mode included in the plurality of first modes is used andinformation indicating whether a third mode that is included in theplurality of first modes and different from the second mode is used,only the information indicating whether the third mode is used isdecoded from the stream, in the non-skip mode, the informationindicating whether the second mode is used and the informationindicating whether the third mode is used are decoded from the stream,and when the second mode is used, the current block is decoded via thenon-skip mode regardless of presence or absence of a residualcoefficient.